Buy what ever brand your more comfortable with and what your buddies have, or what your local shop carries.
The brands are just like cars. Ford or Chevy, toyota or nissan, Associated or Losi. They are all good but everyone has there own preference.
As for turning it up at the house or down at the track, i dont think that is possible, if it is I havent figured that out yet. Just practice throttle control. Your best bet is to buy 2 different motors. 10.5 for the track and 6.5 for your house.
Just my .02, if you go faster than 6.5 at your skill level, it is to fast and more useless than usefull.
Currently I have a 10.5, 7.5 and 6.5 in an rc10t4. I sold the 5.5 because it was so powerful on the track, I couldnt stop spinning the tires, and on the street my truck would wheelie and flip over. All Novak with the GTB and 4900mah lipo batteries.
Not everyone has the same results, that is just my experiance.
Original RC 10T, T4, RC8Te, Tekin RX8. Losi Ten SCTE, Slash 4x4.