amps x volts=watts so the 606i will be good for 6amps on a 2s or 6cell nimh pack but on a 6s pack you could only charge at 2amps(50watts / peak voltage 25.2volts on a 6s pack). So if you are using a 2s of 6nimh cells I would get the 606i but if you are going to run bigger packs I would get the 610i.

I actually have the 610i and a 1210i and they are great chargers. I went with them because I run a converted 8th scale on 5s. I am also thinking of picking up the 606 for when I run indoor 10th scale, so I wont have to carry around the power supply.