I’ve stayed at The Venetian three times, The Mirage, and Caesar’s.
The Venetian is the nicest; the rooms are huge, the bed is comfy, and the bathroom is spacious and well-appointed. The resort is huge so things like room service may be unwieldy (don’t call at midnight on Saturday and want dessert in half an hour), but I don’t recall anything more than minor inconveniences with service during my stays. I’ve stayed at other places in Vegas besides the ones you’ve listed (Bellagio, Wynn, “thehotel” at Mandalay Bay) and think the Venetian is probably my favorite.
Caesar’s was pleasant and the bed was fine, but it was just an upscale hotel room—not really worth a premium price. The room at the Mirage was probably a little smaller than the one at Caesar’s, but pretty equivalent otherwise if I recall correctly.
TI is going to be a step down from the Mirage as they’re owned by the same company and they have a sort of pecking order, but it’s not going to be a big step. Harrah’s is an older hotel; I have no personal experience but people tell me it’s fine as far as a place to sleep and shower goes, but it’s not memorable.
I’m willing to spend a few extra bucks on a room because if I wanted to stay someplace uninteresting, I’d stay at home. Spend your money on what’s important to you for the trip—if it’s food, shows, or gambling, stay someplace cheaper and spend the cash on what you’ll really enjoy. Have a good trip.