I don't have experience with a MIFI device, so I really cannot speak to that. My understanding of it is that it is a hotspot that gives you cellular access to the internet, while out and about. So, in principle, one would expect to be able to use Apple Maps for navigation, provided GPS information is available. However, if the MIFI has no GPS tracking ability, and the iPad software doesn't know how to get that info, I don't think it will work. You should ask if the MIFI device servers as a GPS receiver for the iPad...if it doesn't it won't work. Or ask if good location based info can be obtained from the cellular network by using the MIFI. Most cell phones have GPS antennas in them...maybe the MIFI does too!
What I know will work is to get a GPS receiver for the wifi-only iPad. I have the Dual XPG-150. It connects to the iPad via Bluetooth, and it provides the GPS signal the iPad needs for navigation. Then you need maps either stored on the device or brought down via an internet connection. For this, I have used Navigon or MotionX GPS Drive. GPS drive is the less expensive option letting you buy service for a month or pay $10 for a year. You can download maps to the device. You never need an internet connection since the maps are stored.
The other option is to get the cellular version of the iPad. This gets you the GPS chip. Then if you store maps on the device, you can do nav just fine. You still have to get the app with the Maps. You can buy the cellular plan for the iPad...and turn it on/off when you need to travel and download maps on the fly, or just get a Nav app that provides maps to store on the device.
Also, if you get the verizon LTE iPad, and a cellular plan for it, then it can serve as a hotspot...so you won't need the MIFI anymore....just let your devices hook up to wifi on the iPad. So you can do Nav (with proper selection of app) and connect laptops to the internet via the iPad's hotspot.
I don't know what the Apple Store guy was saying. That all sounds rather bogus to me. Turn-by-turn is in Apple Maps, which came with iOS6, so that may be what he means. But you still need a way to get gps info. If Mifi will do that, great (this means it is either deriving location info from the cellular network or has its own GPS receiver inside)! If not, you need a gps receiver -- either an external one, or one built into the iPad.
So, in short, find out what exactly the the MIFI does....if it has a GPS receiver and can pass that info to the iPad then you're golden. Look it up on the internet...there should be some good info someplace on it. Otherwise, pick another solution. The solution I use is to get the cellular enabled iPad. It makes the MIFI obsolete.
Personally, I'd dump the mifi and get a full up cellular iPad and a shared plan (or whatever plan makes sense).