Staying at the Spa Tower at Bellagio is okay, but first choice should be the main building. It's not the Spa Tower has any major flaws, it's just out of the way, with more distant views of The Fountains & The Strip.. (across an expansive view of an upper deck parking lot).
And the rooms do appear a bit smaller.. the twin bedded room has enough space for a desk & one chair... (no other seating... 2nd person has to sit on a bed)... really not desirable for a hi-end hotel.
If it's a comp it's one thing... but out of pocket, rooms in the fountain tower are the better choice.