Star Fox Adventures is the better game I think. It has better graphics, better music, and it took me about 4 hours to bore me silly, where Sphinx took me 3.
In Star Fox Adventures, what really got to me was the incredibly annoying Tricky, the incredibly slow plot, the incredibly SENSELESS plot, the endless collecting of items just to move on, and the boring repetitive gameplay.
In Sphinx, what really got to me was the sense of deja vu that I was just repeating Star Fox Adventures. After collecting 10 amulets and being rewarded with a key, I was a little unsettled. After hunting all over the damn place for 3 jewels 20 minutes later and being rewarded with yet another bloody key, I simply had enough. I didn't play much longer after that.