in my opinion, it's really just personal preference. From what i heard when i was looking around for a new bike, most people say that shorter people tend to like the compact/sloping top tubes.
Specialized has compact frames and i tried out a couple of their bikes and really liked them, but i didn't like the sloping top tube (even though i'm a short 5'3'' girl). The guy at the shop put me in a size 50, but it felt small, esp. with the sloping top tube, it felt like there was "bike" missing...if that makes any sense. I did end up getting a specialized, but i went up in a size (52) so that the top tube was more straight. It ended up being a perfect choice because the top tube was the perfect length for me even though now with my road shoes and padded cycling shorts, i don't really have much clearance - this might sound weird, but the padding on my cyclings shorts pretty much touch the top tube when i stand over it.
just go test ride a bunch of bikes and go with what feels best! 🙂