LW automaitcally figures out which is better. A Leaf connects to 3-5 ultrapeers; ultrapeers connect to 30 peers (other ultrapeers) and each UP acts as a relay for up to 30 leafs.

General rule of thumb is that if you are sharing lots of files, you are contributing lots already to the network, and so leaf mode works best. If you are best able to share upstream bandwidth, your LW can best help by passing messages and getting leafs connected, the major job of an Ultrapeer

Both a leaf and UP should get about the same quality of search and download results (though I think an UP might have a bit more of an advantage that's not very noticable).