I wouldn't buy the 3800 stricltly due to the reduced capacity (mah) - shorter run time. That may not be an issue if you're racing and the heats/races are short.

I have a 2wd Slash and have been running Sidewinder/5700 combo with a 2S 5000mah 30C lipo and there is no shortage in power. I'm changing to a 3500 Velineon cause there's too much power with the 5700.

I'd buy the lipo listed in the middle, the 40C 5000mah. I doubt you'll see any diff with the 50C and the diff in capacity is small.