Having made 2 films with Shyla and 1 with Aletta, I can tell you that both are delightful and fun women. I can also tell you that Shyla is a bit larger and firmer uptop. She would eventually crush Aletta but it would be an epic struggle. However, Aletta has a firmer ass and would triumph in that collision. As far as sexier, that is a tough one. Both can work it! You better get the scene right the first time because you're going to get drained. But Aletta has that little something extra that can prime the pump faster for those 6th and 7th follow-on performances.
I'll vote for Shyla winning the titfight, Aletta winning the ass war, and Aletta being the overall sexier.
P.S. I'm kidding about working with both of them. Your guess if I worked with only one of them.