I believe Starwood's suite upgrades are space available, but most people seem to think they do a pretty good job of upgrading their platinum members.

Hyatt gives diamond members four upgrade certificates per year - these certs allow you to confirm a suite upgrade at the time of booking. I believe their top tier members are also pretty happy with the upgrades they receive when not using the certs.

Both Hyatt and Starwood have significantly fewer hotels in Europe than Marriott (especially Hyatt), so you might want to check their locations. If they're not where you want to be, maybe that would influence your decision.

Also, I'm not sure what Hyatt and Starwood offer in the way of status matches or challenges. You might want to check that out. If you'll be starting from scratch at one chain, but starting with top tier status (or a short path to it) on the other, that might help you decide.