I think Stacy's face is slightly better looking. However, I've recently taken a liking to enhanced chests, so I like Bella's body more. I also like smaller nipples, and Stacy's nipples are bigger than Bella's.
As for service, Bella feels more confident and assertive while Stacy strives to please you. Technique-wise, both are great PSE/GFE's but Bella is more energetic, while Stacy is more sensual and passionate. I enjoy Bella's attitude more but Stacy's really really good with her tongue.
I personally still prefer Bella overall, but my goodness it is so not black and white.