Ray raises a good point. The 35/2.5, although smaller and less expensive, does apparently provide a bit more contrast. So, there are several factors to consider: size, cost and rendition (e.g., contrast). Mind you, I have no personal experience with either lens (although I do have the 35 Ultron on its way to me), but thought it might be helpful to point out that, from what I understand, there are several factors to consider when comparing these two lens. Oh, and to second another point Ray makes, the 35 Ultron is really not that large. I recently got a chance to check out Ray's 35 Ultron and found it to be a very nice sized lens (heck, that's why one is on the way to me now!). Maybe not pocket sized (I have a 40mm Rokkor for that), but definitely not large and with a very nice feeling in one's hand. Just my $.02 worth mind you.
-Randy