If you don't want to root your phone (worth rooting any phone, in my opinion) I'd personally go with the Optimus. At least, until the Transform gets Froyo. If you do want to root your phone and make it more customizable, I'd go with the Transform. The large keyboard was a big deal for me though. From what I've seen (and played around with in the Sprint store), they're both close the same speed and snappiness with Froyo, if the Transform isn't better. You also get a longer screen and more CPU with the Transform. I don't know about the Optimus, but the Transform has a great community and modders here and on the SDX forums. Vampirefo and Solust and many others are a great coders, and 90% of the Intercept mods work with the Transform, adding to the things that it has going for it.
Ultimately, I think the Transform is much better if you root it.