I have had a two pole sensorless motor setup (traxxas vxl) , four pole sensorless motor (new castle 3800 sct system), and novak sensored motors. The sensorless motors def. make more power but the two pole sensorless motor setup was more of an on off switch but the four pole is just as smooth as the sensored setup. Most sensorless setups also run cooler compared to sensored ( of course this is drasticly effected by your gearing ).
If it were me I would buy a sensored setup for my son so he could get used to it while racing and then move up to a faster sensorless motor when they are ready to move up to a mod class.
Schumacher Cougar SVR, SV2, KAT K1
Kyosho RT5, RB5, ZX-5 Viper RC Solutions
Losi 8ight-E 2.0
Xray T3 '12 (USGT/17.5 TC car)