Is there anything to be said about the two based on stability? Back pain comes after playing for a while; bending knees helps. Comfort is the most important thing, but is there a trade-off?
Whichever works, I think Rempe had the spread eagle with both legs straight, but most players either bend both knees or lock the back leg and bend the front. As far as pain, all players at all levels will develop back pain or just plain wearing out eventually. If you do have a stance that is comfortable and your weight is displaced properly, maybe you need to door some body strengthening. All players have lost matches and tournaments because of lack of conditioning. When Mika won the US Open from the losers bracket, that was a testament to his conditioning, I highly doubt he could of won without.