Have to say I really enjoy the PRO 10, having only played one season of the previous XPress league, I find it very competitive and as a result it's improving my skills at this game, even tho it's competitive coaches like rayski, doc and many other's help to improve newer coaches like myself, which in turn benefits the site.
The four season format is very good as gives more room for developing a team compared to the PRO 8. The PRO 8 is fun but since it's shorter I found it harder to build my team the way I liked, as any slight mistake is very costly.
I voted PRO 10 as I'm dropping one league, as I believe with just focusing on one league on the site, it should help me to improve my game further and I done a few seasons of the GM league on Madden 15 and enjoyed it so if it work's in Madden 17 I would give it a go,