It depends on whether the bike is a compact geometry or not... And looking at the orbea diva geometry, I'd say it's definitely compact because the 53 cm actually has a 46 cm seattube. And at 5'1 - I'd definitely fit the 53 and would find the 49 cm too small for me. With the 53, I'd still be running about a 110 cm stem to make the top tube fit & would have a setback seat post, and seat all the way back.

One of my bikes is an "effective 54 cm" - but the seatpost length center to top is actually about 46 cm, and the top tube is 540 mm... and at 5'1 with shorter legs and a longer torso and/or arms - I love the 540 mm top tube. It also gives me a 73 degree seat angle - I can't stand that small bikes run 74-75 degree seat angles.

As for the standover test - it's not so important on a road bike. If you can stand over it without absolutely racking yourself, that's fine. Rivendell's website describes standover clearance using the distance from the pubic bone & doesn't count the soft tissue. Because I need/like a slacker seat tube angle, I generally buy the biggest frame that I can fit myself on - with a 73, I still run a setback seatpost and have my seat all the way back on its rails. So I'm willing to sacrifice standover for nicer fit.

And I definitely don't ride women's bikes 'cause I don't like the short top tubes.