The main difference between the 2 frame styles is standover height. As a normal frame's top tube doesnt rise, the standover height is best measured according to the seat tube length. If you're suited to a 56 seat tube length on a normal frame, thats the size you would buy. On a sloping frame however, due to the top tube sloping upwards you'd need to go a size or 2 down and go for a 54 maybe as the middle of the toptube would be higher up than the seatube therefore increasing standover height.
However, any cyclist who knows anything will advise you to go to a bike shop and try the bike out for size first as what may look good on paper may not necessarily be good in realistic terms.
Smaller frames are also a bit easier to handle so worth bearing in mind