Instead of a LHR-DXB loop, why not LHR-MCT? MCT seems much more interesting to me than DXB (unless your into glass and steel). You also get a few more miles.

Depends on what your after in the end.

Both will be incredibly HOT in the middle of summer.

Making DXB the better choice as it is easy to stay in air conditioning there. The heat is astonishing and really affects activities. Does make for some very cheap hotel rooms, though, particularly at the high end.

True, but there are much more interesting sights in MCT. As I said, it really depends on what you want out of a trip. If you just want a beach and nice hotel, go to DXB. If you want interesting places to visit and varied landscapes etc, MCT is better.

So... if the OP is taking his wife, I would go to MCT, if I were just going to do an MR, DXB is better due to more flights etc.