Mp 20 all the way the, Mp20 you can also upgrade to the MP40 with just a kit, And the mp 10's outer housing shatters if falls off the tank and is not replaceable.. I got this info from a co-worker and he read it from nano reefs..
I say MP10. I know the MP10 is perfect on a tank your size since I run one on a similar size tank. If you choose to go with a 60 later down the road I would rather use 2 MP10's (one each side) than one MP 20 blasting from one size.
I know I am getting into this a little late. I am glad you bought the 10. I have it on my solana & I have it dailed all the way back. Most of my corals are LPS & softies. The 20 would kick the crap out of them. I have my MP10 dialed all the way back.