Would it be better to mount the side rear speakers up about ten feet, or low on an end table. I only have these options as there it a big picture window in my living room.
Depending on your speakers, 10' high up on the sides, facing each other across the room, with a slight tilt down, will work fine. (Typical installation @ your local Century AMC Cinema Theaters)
My JBL S26's Right / Left Side Surrounds are 6' high, 3' feet back from my 'sweet spot', facing each other across the room with no tilt.
FYI: My Rear Center JBL S26's (pair wired in-series) is 8' high with a slight tilt down, centered between the beam, about 5' apart in my 20' x 30' HT/family room.
If by end table you mean, the speakers will be very close to the listening sweet spot? That makes for very difficult good calibration. Of course the tweets at ear level, yet properly spaced is best, so anything else is going to be a bit of a compromise.
I like wall mounted surrounds at cieling level myself. But still almost impossible to say which would truely be best and probably is going to be dictated by the room characteristics more than anything else.