Sounds like you just need a PAYG micro sim, which is what I use for the occasional times I'm away from home, ie . holidays and short breask ect.
Choice of which network is difficult one. My wife and I live out in the sticks of Herefordshire, 17 miles from nearest main town, so obviously one has to take into consideration if you want your 3G to be accessible at your home location, which for most people would be a resounding yes...lol.
Fortunately our local exchange in the village was upgraded to ADSL2+ last November so we now get decent net speeds, but 3G signals here are not so good, users on o2 and a couple of others can barely get a signal here, but T-Mobil gives a signal strength of 2 bars and Orange gives pretty much full strength with all bars showing.
So that's the main reason I have a T_Mobile monthly contract on my iPhone, same with my wifes Galaxy S2, and of course why I use a PAYG T-Mobile sim on the iPads, plus the fact you can use the Orange network too which is a plus if your a T-Mobile customer.
We also have occasional weeks or short breaks in a caravan in Wales, and that also has reception for our network. I've managed to get pretty consistant reception where ever we have been in the UK including northern remote parts of Scotland, so although it's a little slow but probably the norm for 3G I've found it does the job. I just pay for the amount of time I need.
We are retired, so PAYG is ideal for us, but I may be using it even less now.. since I discovered I can tether my iPad or anything else to either my Iphone or the wifes Galaxy S2 as both have Tethering Hotspot options, so I can use that instead which probably makes more sense, as I have to pay the monthly bill for those anyway.
I've never heard of gifgaf...so no idea on that one.
At one time o2 would have been considered the best, as o2 is BT of course, but these days, I don't think there's too much between any of them, coverage wise anyway, though you will still get the odd location where reception is poor, and AIS our home location is not good, so choose a network that works in your location, as it can be handy in the event your main net provider ever goes down.
My son has a o2 monthly deal with his iPad, and the speeds when loading web pages when I played with his iPad a couple weeks ago seemed comparable with my own, if anything I would say his o2 3G seemed a touch faster, but that could be because he lives in a town and not out in the sticks, and that was when comparing with my iPad1 which he also uses, so not sure if my new iPad3 on 3G is any quicker... can't say I've noticed much difference TBH though early days, I have noticed however that Wifi on the iPad3 is definately much faster and instantly noticable when compared with my iPad1.
I like the fact with T-Moble you can also use the Orange network, so it doubles the chance of getting a reasonable signal, they are competitive on price though they have just increased my contract charge by 60p a month..doh. but at least I have unlimited data, so I don't have to worry about data limits, thats on the phones, the PAYG ipad sims do have a limit but I doubt you would ever exceed them, and if you tether to the phone.. again there's obviously no limit.
In conclusion.. go with whoever is offering the best PAYG deal, and preferably one that works in your home location.
Whats the problem with 3 ? I have used them in the past, but they have no coverage where we currently live.
(another 2 line reply which turned into a book, and is available at all good bookstalls....lol)