I've used both and currently have settled on Xscope. HD Dolphin is definitely slower. Run a test by opening the same site with both browsers and you'll be surprised at how slow Dolphin really is.
I also agree that while the GUI of Dolphin is nice, it simply takes up too much room. With Xscope, the tabs are much smaller, and they don't even appear if you only have one tab open. In fact, you can hide everything (address bar, etc) with a simple swipe up, and show it again with a simple swipe down. This makes it really nice - especially when browsing non-mobile versions of sites.