Alfred is simpler, and noticeably faster than a non-fine-tuned Quicksilver in my experience. And the little details are done better - e.g. punch in an equation and it'll show you the results as you enter them, and hit enter to copy the results. Small, but I use it often enough that I would miss it.
For anyone with a fine-tuned Quicksilver install, you're basically weighing the more advanced features (e.g. "find file -> open with -> find application" is a favorite of mine) against Alfred's "Power Pack" - it'll cost you $ and probably a bit of speed to switch to Alfred. Ultimately though, a highly customized Quicksilver install has basically no competitors.
For anyone with a fine-tuned Quicksilver install, you're basically weighing the more advanced features (e.g. "find file -> open with -> find application" is a favorite of mine) against Alfred's "Power Pack" - it'll cost you $ and probably a bit of speed to switch to Alfred. Ultimately though, a highly customized Quicksilver install has basically no competitors.