both are perfectly fine
that's personal. 2x31 3x25+, I usually do 2x31 3x27 or 29+, do the job just as good. Obviously 5x31 will be better competitively, but most of the time, you won't notice.
Sadly Ubers is something we really don't have in pokemmo, you can however still use them in doubles. I'd advice you to play whatever you feel like playing besides NU that currently is chaos.