After spending a couple weeks in Europe in hotels from SPG, Marriott and Hilton, I hate to say I was a lot more impressed with the Hilton (where I usually stay least) in terms of quality, service and price. I found the SPG properties to be more expensive while not having anything over the other two in terms of quality, and in the case of a couple of Sheratons, way less. I also found extras like minibar and food prices to be ridiculously high compared to those in other brands.
I still like SPG best for points as they are a LOT more valuable than Hilton's and the no fuss late checkouts are a big plus too. Marriott's new AC association is quite nice and a good value, but Marriott is a lot tougher to keep status with.
The lounge/breakfast/internet access is pretty much the same for all 3. The lounges in the Hiltons this last year have been the best by far.