Personally I would get the Mega 16/15/4.It's a nice performer on 3S,not ballistic,but fairly quick on a 6x5.The nice thing about it is on 4S,it really comes alive
It is a good solid motor that you can use in many applications.Mine has been in about 5 airframes so far.
As for the Nimh batts,I don't use them any more,so cant really comment,but it will definately work,just as you say with less volts = less rpm.
Cheers,
Bleary.
ps make sure you set your low voltage cutoff for the batteries you are using.