Also, Quicksilver only indexes the things you tell it to, whereas Spotlight by default indexes the entire system. For certain searches this leads to a lot of chaff in the search results.
Spotlight (and pretty much all Apple software) is rarely explicit about advanced (?) functionality but quite often there is some depth.
My favorite example (also part of Spotlight) is the calculator. It seems simple but it is capable of quite complex calculations. Try using sin, pi, e or sqrt. (If math isn't central to all you do this calculator is the only one you ever need.) You can also copy results by pressing Command-C. None of this is obvious. I didn't know that you could copy results for years, despite always complaining about the lack of that feature whenever I talked about the Spotlight calculator. (I was finally corrected that you can indeed copy results here on HN. Thanks guy/girl!)
I don't know whether that's a good or bad thing. I personally like software that doesn't try to please advanced users at every turn, but that's more about aesthetics than usability and I wouldn't try to generalize it.
I think you can guess from this that I don't use Quicksilver. Spotlight serves me well.
however, i much prefer quicksilver; it does a lot more than spotlight.
It also led me to my other issue with Spotlight - it can and will update it's list of found items, even changing which item is selected.
I've experienced this race condition quite frequently, and it's very annoying to have the right thing selected, then Spotlight swaps it out from under you and you've just opened something else.
QS doesn't have this problem, as it only changes the search results after receiving user input.