Actually, the weight of water doesn't matter at all... Since it is an above ground pool and the equipment is above ground on the same plane, you have a net of zero in regards to head of pressure (water in the pool looking to get out) and the water you are pumping back in (within that same height of the water column that is the source)...

But, that isn't the issue... The issue comes down to the efficiency to be gained by pumping hot water into the coldest area of the pool (the bottom) and letting it mix and rise through the rest of the pool, compared to pulling water from the top of the pool (the warmest area, especially with a pool cover on) and returning heated water within a few inches of that area (at the eye-ball return). With multiple returns, all pointed to the deepest area, you will get a fairly decent mix as well, but ideally it wouldn't be "as good" as returning through the main drain (which is the lowest point in the pool).

In regards to solar heat, that is a completely different discussion, and in my case, not viable, as I have a cedar shake, heavily pitched roof in an area that would complain up and down if I put something so unsightly on my roof... So, solar is out, and the gas heater I have I figure will only be used at the beginning and at the end of the season, so it's overall cost isn't a big deal.