nashman,
As an instructor for the last 6 years I have seen a lot of people get into too much plane as a second plane. I know that there are those that are going to come on here and say that the UCD would be a great second plane, but I disagree. Maybe a 3rd plane, but for a second plane you need a plane that is a "step up" from your trainer. That means you want a plane that will do more than your trainer, but still doesn't have bad habits or tendencies. The UCD is designed for one thing, and one thing only, to fly 3D. Which means that it has big control surfaces and can get very wild in a heartbeat. Plus, the UCD's have very tender landing gear which can get ripped out if your landings are a little rough. The Sig 4-star planes and the Goldberg Tiger II's make great 2nd planes. They are very nimble meaning you have plenty of plane to use as your skills get better, but they will tame down and land like your trainer. My advice would be to wait on the UCD.

That's my 2¢ worth

Ken