I'm doing the same sort of analysis. I'm Platinum Hilton and the treat me very well. After that AMAZING Q4 promotion (4 stays == free night certificate good for ANY property) I'm very, very happy. I end up getting about 15 points per dollar which is quite impressive. The properties vary quite a bit from dingy (Hampton I'm looking at you) to impressive (nothing has topped Hilton Athens). Good news: there are tons of properties so no matter where I go there is a Hilton. Makes it easy.
With SPG - you can get up to 3x points per dollar. Sometimes they have run a promotion where it will be up to 9x points.
I like to redeem my points internationally (usually better bang for the point and lots of fun). A category 5 hotel in Brazil (the best they had) costs 12k-156 SPG points depending on season. The top Hilton I found in Brazil costs 35000 points - almost double SPG. I have found SPG properties to be better than Hiltons on the whole. They are also much more "unique."
New features at HHonors - cash and points is now offered! Plus you can pay more points to get upgraded suites! Finally!
Tough call - anyone else want to weigh in?