There are numerous threads dealing with these issues but a noob may find it all very confusing. The following is hopefully a clarification for those who are terminally confused.
() US vs Other Locations. What follows is information specific to the US market. Users in Europe, for example, have other options, especially in terms of data plans for "mifi" devices and/or data plans. It is risky to assume that information from European users applies to US customers and vice versa. In general, European customers have more and less expensive options. Canadian users typically have options somewhere between those of US customers and Europeans. On the other hand, non-US consumers typically don't have 4G options at the present time. In comparing your options with those of other posters, always try to determine their location. That will go a long way to avoid confusion about your options.
() 3G or Wifi. This is the first decision purchasers must make. A 3G iPad is about $130 more expensive than a wifi model (in the US.) That additional charge buys (a) a "non-contract" data plan from a cell phone carrier (can be activated/deactivated on a monthly basis) that typically runs about $30 per month with a ceiling of 2Gigs of downloaded content. Additional downloads can be purchased for about $10-$15 per gigabyte. If you (a) need 3G access occasionally and/or (b) have no other options for internet access when wifi is not available, this is the simplest and possibly the least expensive option. As a bonus, it provides built-in GPS functionality (more accurate than wifi) for location information. But keep in mind that if you use even a single download byte during a month, you'll pay for the month.
() Wifi with Mifi. If you opt for a Wifi only iPad you can still get 3G (or 4G) internet access via a "portable hotspot" that connects to a cell carrier's network and provides a wifi signal for your iPad. This is typically a small device (about the size of half a deck of playing cards) that provides 4-5 hours of connectivity on a single charge. In the US Verizon, AT&T, and Sprint all offer such devices (on a 1-2 year contract) and other carriers offer pay-as-you-go options. There are advantages and disadvantages of the mifi approach.
On the plus side, you can opt for 4G access. This can be 7 to 10 times faster than 3G coverage and even faster compared to some "public" wifi sites (e.g. hotels and coffee shops) as well as being more secure than the latter. But be careful. All "4G" networks are not the same. Sprint's "4G" network is much, much slower than Verizon's and AT&T's. And Verizon's 4G/LTE coverage is much, much wider than AT&T currently offers. Another advantage of a "Mifi" option is that you can connect several devices (even simultaneously) to the portable hotspot. If you need internet access for both a laptop and an iPad, for example, this can be a significant advantage.
As far as cost is concerned, you'll have to purchase a mobile hotspot (typically about $50-$100) and (probably) a long-term contract. This typically runs about $50 per month with a five gigabyte ceiling. That's more expensive than the 3G iPad option if you use the hotspot only occasionally. But if you travel a lot (i.e. every month) and stream audio and video content when traveling, the hotspot may end up being slightly less expensive over a couple of years. (I travel one to two weeks every month and typically use about 4-5 gigs of downloads for my iPad and my laptop. For me, it's a far better option than a 3G iPad.)
On the minus side, a mifi hotspot typically lacks the built-in GPS functionality of a 3G iPad. But if that's important to you, you can purchase a separate bluetooth GPS receiver for about $80 that is both more sensitive than the 3G iPad's built-in GPS and can be located separate from the iPad to provide the best possible reception. (Just search for 'GPS iPad' on Amazon.)
Still another disadvantage of the mifi option is the shorter battery life of the devices compared to the iPad. Using 3G (and especially 4G) will drain the battery of a mifi device or an iPad. But because the iPad has a larger battery, it will typically last a couple more hours than a mifi device. That may not matter to you if you have access to power while surfing the net. (How often are you going to use 4G/LTE access when you're away from a power source?) But it is something to keep in mind.
And one other caution. If you get used to true 4G/LTE access, you're likely to be quickly spoiled with the speed. (Much less likely for carriers whose "4G" networks are not "LTE.") But it's also the case that if you want to have 4G data speeds on your current iPad, this (or tethering below) is the only way to get it. The iPad 3 may offer 4G speeds with some carriers, but that is (imo) unlikely. More likely is that you'll have to wait for at least another year for 4G on the new iPad models.
() Tethering. If you already have a data plan for your smartphone you may be able to tether the iPad to the phone and use its internet access. If you have (or purchase) a 4G/LTE phone you can even get 4G speeds from that connection. (No workee for iPhones. Only Android.) Typically, this costs about $20 per month from your carrier and increases your monthly ceiling on your phone data plan by 2 Gigs. (If you have an "unlimited" data plan for your phone, that may not matter.) You may be able to avoid the additional tethering charge if you jailbreak your iPhone or "root" your Android phone. But doing so will violate your contract with your carrier and may lead to undesirable consequences if the carrier determines you are tethering your iPad to the phone without paying for the privilege.
By the way, if you're unsure about how complicated "tethering" (or use of a mifi hotspot) is, don't be concerned. The procedure may vary slightly according to your phone/hotspot, but in general it simply requires that you "turn on" tethering on your phone via the "settings." Once that is done, the iPad (or any other wifi enabled device) simply sees the phone as supplying a wifi connection. With a hotspot it's even easier. Just turn it on. The only thing to remember is that active tethering drains the battery on your phone. So turn it off when you're not using it.
Depending on your carrier and your data plan, you may be able to activate/deactivate tethering on a monthly basis (analogous to the 3G iPad data plan.) If so, you'll typically save both the $130 on the purchase of the iPad and $10 per month when you activate tethering. Be sure to check with your carrier about the details of tethering, especially if you have an "unlimited" data plan and want to hold onto it. The carrier may well try to force you to shift to a more limited data plan in order to tether your iPad to your phone.
Hopefully this is helpful to those who are lost in the jungle of trying to figure out options for internet access on the iPad. No guarantee of accuracy but I've been wandering around in this jungle for quite awhile and the points above should be accurate as far as most US carriers are concerned. Of course, corrections of errors above are always welcome.