I don't know anything about the spas; however, in the past have liked to stay at the MGM properties. The resort fee, however, is a KILLER!
In the past we have stayed at MGM, Luxor and Monte Carlo. Love MC for the location (connected by free tram to Bellagio, which is a favorite for us to play BJ on the machines) ... but this December, we are planning to stay at Bally's. It is also in a good location but the parking garage is a bit of a hassle compared to MC. We've stayed at Bally's twice before and the room was fine. Not as nice as the MGM properties but the price was right. I can get reasonably priced rooms, but when you add $20/day for the resort fee - it just kills the deal for me.
One of the main things about Bally's and the other Harrah resorts is that they don't have many (what we consider to be) good gaming options. All of the machines have crappy pay tables and/or it is hard to find low-limit, good rule BJ. I'd love to give them some business, but just can't find "anything" I want to play.
We currently are holding a four night (Fri-Sat-Sun-Mon) reservation at Bally's (note two nights are during the weekend) and the total with taxes will be $250. From there, we will move to South Point which is about 4 miles south of the airport. Really love the casino and during the rodeo (when we go) is cowboy central because of their equistrian center!! We actually do most of our gambling at this casino, but like to be able to walk about at night, so spend most of our nights on the strip (after putting the car away). We tend to get good comps at SP ... they comped several of our meals plus our two nights stay when we were there last December.
Based on my experience, if you are budget-minded and want to stay mid-strip, I'd recommend Bally's.