Wind does a better job on providing signal to to commercial coverage such as malls and stores, this is mainly due to the repeaters they have installed.
However, Mobilicity does a much better job in the residential side. For example in my house and other houses I've visited and compared, I always had a better signal with Mobilicity than Wind. It's not that Wind didn't always work in the houses, it just wasn't as good as Mobilicity.
Voice quality wise, Mobilicity wins hands down. Mobilicity provided crystal clear voice while Wind, it has gotten better, does randomly suffer from half-duplex issue which can get annoying.
In the end I did switch to Wind due to better commercial coverage such as at my school and malls, and to the fact they are geared to be next national carrier and being that my plans were the same price it didn't hurt to switch over.