Stayed at Wynn Dec 2007, Palazzo Dec 2008, and Encore Jan 2009. All basic rooms with a "$20 trick" upgrade to a high floor view of the strip in both Wynn properties and a mid-level floor view of the strip in Palazzo.

Favorite room by a little bit was Palazzo. Not sure of the dimensions but seemed more spacious with the sunken couch/desk area. The Encore room had a fair sized couch area also.
The decor is a wash as all three were very nice and modern. Personally, I liked the Encore's decor. LCD tv's in each with HD channels. Large washroom in all three (dual sink, tub, shower, separate toilet, etc). Wife loved the toiletries in all of the bathrooms. She really liked the bed in Palazzo. Not sure why but I liked the one in the Wynn the best (tho, I suspect it's the same as the Encore). The wife was really bothered by the noise from a vent in the room located over a ledge near the ceiling in the Palazzo room. You can't control it (we called down to the desk to ask). We just ended up muffling the sound with a pillow.

I found the red decor a little excessive in gaming area of Encore. Found drink service to be very good even late at night/early in the morning.
Didn't enjoy my video poker experience in Palazzo. Slow drink service and non-existent after 1am. Had a VP machine crash on me too.

Didn't go in but found the pool areas at Encore and Wynn (you should have access to both I think) more interesting than Palazzo's. (Didn't check out Venetian's which I also assume you would access to both.)

Parking for all three were fairly convenient and a relatively short walk to the room elevators.

Service-wise, surprisingly(?) I felt a bit more of a friendly vibe at Wynn and Encore (though our interactions with staff were primarily limited to Front Desk, security, and CW's. YMMV of course.