the only difference is in th apammping, generally Garmins mapping is better, but now tom tom and garmin use the same mappin provider, so the new tts and garmisn will me much the same.
i have a garmin i3, an old model but it still works great, only has 3 buttons on /off, back and a wee scroller thing, only cost me like £70, and you could get them recently for as little as £80 new with european mapping and speed camera pois are easy to get pocketgpsworld.com is great only £19.00 a year, for unlimited downloads of the database so between 3 users(me dad and bro) its only £6,35 each, but they have been discontinued in favour of the nuivi, which are a fair bit pricer, twice as dear, but, only as expenive as the base tom tom one, which i got my dad for chrimbo and it had no problems getting a GPS signal any where, middle of halford, in the kitchen etc.
might get a more up todate one in the new year.