Personally, I think a sloop is easier to sail "in the groove" than a Uni. And when the wind comes up, you can spill the main while keeping the jib pulling, so you don't lose as much drive in a big gust as a Uni who has to dump a lot of mainsheet or make other adjustments to de-power, and as Matt said, the drive is down lower in the jib so you don't fight the righting moment battle as much when it's windy. A Uni will outpoint a sloop but a sloop will out run a uni, (go faster but lower) upwind. On a reach, there is no question the sloop is faster, same hulls, same crew, same sail area.

I find it much easier to watch the jib tell tales when driving a sloop than looking up at the main when sailing Uni, but I have more time on a sloop than a Uni so I really have not figured it out yet.