I mostly use Safari on my iPhone and Atomic on my iPad. The reason is that I need to go to a work site that knows how to deal with the iPhone Safari browser, but not with the iPad. Atomic lets me set it up so it identifies itself as the iPhone Safari browser and everything works.
Besides that, the rest of the features are mostly a matter of preference. Atomic also supports private browsing.
The best thing about Safari is being able to keep by bookmarks synchronized across my iPhone,iPad and Mac.
For me, I can sometimes notice rendering speed, but most of the time there's not enough of a difference between browsers to really matter that much. I guess that depends on the sites that are important to you.