I really like the Ace 2, i'm only waiting for ICS to come to this baby and then i will purchase it. I don't like the Xperia U only because of the lack for memory expansion, i don't like the Sola because it doesn't look that good.
I had to choose between Ace 2 and Sony Xperia Sola. I choosed Xperia Sola, I think it is sligghtly better.
I think together with the mentioned xperia u, the sola is also a strong rival. It offers similar advantages like the u, and with a few bucks more eliminates the lack of microsd card slot. I think the sola is a little better than the ace 2.