I like Google sync, because its a full fledged exchange server, and WinMo has a built in exchange client that works really well, and doesnt need a separate app installed.
MS My Phone is cool cause it will sync photos, videos, docs, text messages, you name it. They offer 200 meg of storage. Google Sync will only do calendar, contacts, and email.
I went with the MS one. I really love Gmail an d I use Gmail and Google voice but I like the idea of txt and photo management.