I had a similar dilemma as I was in the middle of medium and large. The M/L/XL is meaningless as different manufacturers at different times use different designations. Pay careful attention to the stand over of the bike as its very similar if not identical on some sizes. Perhaps reach should be your main consideration? I am longer in my top body (arms) than in my legs) so a medium frame can feel crowded. In the end I went for large. The stand over between M & L is the same. It's not perfect, but that's the problem with off the shelf sizing of bikes.
Good luck with your decision, and hope this helps.
Good luck with your decision, and hope this helps.