I picked my inject based upon how fast I wanted to go. I'm running 50s because hopefully I'll get into the low 11s (high 10s on a very good day ) 42.5s run out at mid 11s from what I've read and heard.
Tunability will be up to the person burning the chip.
My car is mostly street driven and I've had no problems with streetability.
With the Maxeffort, you can tweak to your hearts desire, whether its 42's or 50's. Both should run the same. If the 42's are big enough for you, I wouldn't change to 50's.