I've currently got Next G and 3.
Next G is pretty expensive compared to 3, as 3 was offering $14.95 / month / 1GB deal when I signed up, whereas Next G costs me $49.95 / month / 200MB (I was on CDMA/EVDO deal before so I got it $10 / month cheaper). Next G works faster and has a better coverage than 3, but as I don't do heavy downloading etc, I don't find the speed difference too severe.
I can't comment on Vodafone as I haven't tried it (although I have noticed a lot of people using it in QP - is there any reason why they'd rather use Vodafone one rather than the QP freebie apart from security or not knowing that there is a freebie one?).