Can't help with your choice as I'm not on the network but bear in mind that if you decide that you don't like the network then you're pretty much locked in.
I've read elsewhere that the SIMs are not compatible with GSM types as they use a different voltage. This means that you can't then use the phone with a GSM SIM, even assuming that the phone can use the GSM frequency allocations.
wouldnt bother with them mate they are the worst network going i have had to exchange my phone a total of ten times they have also taken me off contract due to me taking them to court and not only that i am still recieving a phone bill s for a phone which i dont even have for the 6th month running really bad customer services if anyone is having any diffilculty with this network then ring this number 0870 733 0295 this will get you talking straight to one of the executive offices
I recently signed up with three on the above deal you mentioned (though I think that the deal you mention expired at the end of April). I got the latest and smallest 3 fone, the Nec e616. Its a really good fone, plus you get free football highlights for a month!! there is a problem with the sim card as PaulR mentioned meaning that you could only use another three handset if you decided to change fones. With regards to network coverage, i have my fone set to 2.5G which means that it is not capable of streaming video on this setting it will always be capable of making voice calls and text like a normal fone, i have never had many problems on this setting and if I have ever lost coverage, my fone just roams over to o2 or any other UK network.
I,ve had the moto A835 for 3months now on video talk 500 add unlimited (extra 10 quid) no probs with signal ect.Personally i think its the best phone on 3 at moment best standby ,talktime ect.also has 64mb of internal mem.and bluetooth downfall is they are big (brick)as most of 3 phones are because of what they can do, suppose they will get smaller..I do like the new L G model though no bluetooth....