I don't have either but want to know the details...
I thought the Silhouette would be better, because you don't need to buy cartridges. Is this true? Is it easy to just use the computer to find lettering and pictures, or are the cartridges really the better option?
It depends on:
1) how much time you want to devote to learning up front
2) how much money you want to spend in the long run
3) if you care about warranties
4) how mobile you want the solution to be
Regarding #1 above (learning time), the Silhouette will take you slightly longer to learn - even if you are comfortable on the computer and most image applications. If you just want to get something that you pull out of the box and start cutting - go with Cricut. However, a little bit of time up front and you can really start manipulating files and making new files for use.
Regarding #2 above (money), the Silhouette may be cheaper if you already have a lot of computer fonts. If not, you can still obtain these. Additionally there are some software solutions which bypass the need for cartridge when hooking to your computer. However these are all not supported by Provo Craft and thus voids your warranty (see #3 above).
Regarding #4 above (mobile), unless you have a laptop and a lot of space at crops, the Silhouette is not a mobile solution. This is the main reason I bought a cricut (as I often go to a friend's house to do my day-to-day album). When I go on other crops, for which I plan pages, or am scrapping at home, I never use my cricut. I always use my wishblade and love it.
One other thing to consider is that the wishblade & silhouette use up a bit more of your cardstock supply... I am not a scrap fiend - if large enough I usually just throw in a box and donate to my friend's class supply box (she is a teacher), or throw in the recycle bin. I know other people here on the boards love to use their scraps and really appreciate the paper save option on the cricut...
HTH! If you have any specific Q's on the silhouette, let me know & I will answer as best I can.