so depending on the bike, you're probably best off looking at a 50cm frame. not sure how that translates to S/XS in the bike you're looking at. Based on the sizing chart for the bike you're looking at, I think the XS might be a smarter choice.
standover is pretty important because if you have to dismount in a hurry and your feet hit the ground something else might hit that top tube... just food for thought. Top tube is also important but there are more opportunities for adjustment. Example, you can easily make up the 1.5 cm difference in the top tube by getting a 1cm longer stem & adjusting your saddle .5 cm FWIW, you might be able to get it right just by adjusting the saddle.