Every upgrade has benefits from reducing injuries to getting extra boosters, more seating meaning more revenue!, the main ones to do first are first aid, gym, stadium/seats, then move onto car parking, pitch, flood lights etc!, the best way to do it is to build every aspect for that level!, alot off them are locked untill you complete nearly all on one level anyway!, DONT think O il just keep upgrading my staduim/seating so I can get high attendances because that will come back on you if your not carefull! , meaning you will not be able to fill it during games so you will lose so much bonus!, also gradually raise your ticket prices as you go up the levels, if you fing you are not filling the stadium regular then drop you prices a little till you fill it again.