I have very limited experience with coaching either of the races you are considering. However I'd like to suggest that you do not choose your race based on what is best (unless you're one of those hopeless I-MUST-WIN kind of players) but look at other things as well. With that said here's my views on the 3 races:
Humans - nice all-round team, lots of flexibility which makes for a diverse and interesting game, one of the only teams with ogre acces which is a big plus (IMO the new ogre-minis are SWEEEET), the Griff Oberwald (human star-players) mini is also very nice.
Orcs - also fairly flexible, while definately being a strength/bashing-team they're not half-bad ballplayers either, lots of entertaining rules especially if you are allowed to recruit stars/secret weapons (Throwing bombs and goblins are barrels of fun IMO).
Chaos Dwarf - a very strong bashing-team, limited ballhandling capabilities, bashing can be very fun but can also make you somewhat unpopular

(when you kill the 5th elf player, you can expect very grumpy elf-players), IMO the chaos dwarf minis are ugly beyond compare.
Overall I'd go Orcs or Humans - I actually considered both for a new team I have coming up and ended up with Orcs because I wanted something different from the elegant Dark Elves I usually coach (this of course doesn't really apply to you).
My 2 cents, and have fun with your new league.
PsychoNiko