I used to use Atomic, but ended up switching to Perfect. I actually like the UI a lot better, and find it's got a lot more little tweaks, like shortcuts to remove the address bar, tab bar, etc without going to the entire full screen option.
Another thing I like is that the action icons at the bottom don't change depending on the view. With Atomic when you change from full screen to partial screen, the icons change position (particularly bookmarks/history).
Oh, and the scroll on Perfect is very handy. With a long webpage you can get down to the bottom in no time.
One thing that Atomic has over Perfect is multi-tap gestures. Perfect hasn't really got these down, but they are slick with Atomic. However, Perfect's tab bar is much better than Atomic's and I think it makes up for this...