Okay, for T610(or T616), you definitely need to upgrade to the latest firmware in order to get better RECEPTION as well as louder volume. Now, T610 is GSM 900/1800/1900, but T616 is GSM 850/1800/1900. If you plan to use this phone primarily in USA, T616 is the one to get(Cingular has this model, and is usually unlocked by default so you can just put your TMobile SIM in there). If you do travel to Asia/Europe a lot, then T610 is the one to get since the GSM900 band is still very crucial.
As for V600...It was plagued(same for V400 as well) with firmware problem and slow UserInterface speed. But it's QuadBand, and if you're willing to upgrade to the latest firmware, it'll serve you better. VoiceQuality is very good, and Reception is not bad, either. Honestly, I'd go for V400 instead of V600...since I don't really need BlueTooth that much. I wouldn't bother for V300, though...that lack of 850 Band can be crucial.
One glitch for V400, V525, or V600 is that the onphone earpiece is just not loud enough, and the bluetooth headset isn't, either. However, the ordinary handsfree(I choose MOto OEM) can give you very good voice quality as well as volume. This is why I feel that you just have no need for V525 or V600.