First: I really learned to like QuickSilver, it just makes work so much faster and since I am a guy, that doesn't like to waste a single click I right away was convinced by using it. So now the long awaited Tiger has brought me Spotlight. The QuickSilver guys will have a hard time now, I guess. But after using Tiger's Spotlight for a couple of days (and QuickSIiver less) I am not convinced, that the Apple guys have done it better! There are quite a couple of things that are annoying and simply worse in Spotlight.
QuickSilver is much faster finding things. I am using it mainly to launch applications, to search contacts ... simply for all the things it is made for :-). And the search result (mostly exactly the one I want) appears within the first 3-4 letters I entered. I.e. starting Firefox just costs me 3 clicks. I used to click CTRL (which doesn't work since Tiger, grrr) now I click ALT+SPACE, type "Fi" and the Firefox icon and the action "start application" appears, I hit Return and Firefox is on it's way. Easy, cool guys!!!
Now using Spotlight. I hit the keys APPLE+SPACE (why the heck don't they allow to only use the CTRL key?) I start typing way more letters than in QuickSilver, for multiple reasons. First because it is much slower showing any search result at all, second because the Firefox icon doesn't appear within two clicks, and that simply is tempting to help the search by giving it more letters. Even when Firefox appears, it still takes me one click DOWN to get to the "top search result", which finally is my Firefox application, and click Return. This takes much more time than using QuickSilver.
Spotlight doesn't seem as intelligent yet. When Spotlight starts the "Firefox"-search it first always shows a file "firefox.info", what the heck do I care about such a file (I don't even know what it is good for). QuickSilver (seemingly) shows most relevant results first, like applications you can launch. And I noticed too that QuickSilver also optimizes the order and the first result that it shows, according to the action you triggered last time when you searched the same or alikes. As far as I understand QuickSilver it ranks the results by usage. If you search for Firefox and click "launch" every time, you can be sure the action "launch" is shown first.
Since the Tiger installation I can't use the CTRL key for opening QuickSilver anymore I don't know why but the CTRL key is not useable with QuickSilver anymore :-(. Yes, I remember I updated to the newest version (in the about dialog it says "2A75", don't know if that is the version number) and if I remember right, this version way Tiger compatible/optimized. And since that and the Tiger install the CTRL key doesn't work anymore for launching QuickSilver. Sad thing!
QuickSilver speaks more languages. My system is in German, sometimes I happen to search for something using the english or the german word, like i.e. "screensaver" or "Bildschirmschoner". Using QuickSilver both languages work fine and they both find the system preferences panel. Spotlight only finds it when I type the german word. Another plus for QuickSilver.
So what is there good to say about Spotlight? Mmmmmh. Don't know. I hope, that it is more investigative when it comes to search text that is inside of documents, like OpenOffice files, PDF's etc. But I can't tell, even though I have the feeling that it is better. What I noticed, is that it passes the search phrase onto the application "Preview" when you open a PDF file from Spotlights search result list.
Spotlight also allows to search in the system preferences, which is really a great feature. You can "search" for example for "screensaver" and you get to the panel where you can modify it. But I just realized, that QuickSilver does too, just didn't use it yet :-).
QuickSilver is in my eyes still the better tool. Faster in searching and finding and it also needs less clicks to get work done. If the CTRL key would just work for launching QuickSilver, I would almost be tempted to search for an uninstall option of Spotlight, since I wouldn't need it.
Update:
Some interesting links
Apple's Spotlight hints
Another new Tiger feature compared: Dashboard vs. Konfabulator