Generally, if you are between sizes, go for the smaller option or pick another brand that fills the gap.
If you go for the smaller one, you have more standover clearance, but do check the length (reach from saddle to bars) against your current setup. If you dont like your current setup, decide how you want to improve it and use that as your benchmark.

When measuring a bike, there are all kinds of angles that are almost impossible to measure, esp with a suspension system. I suggest you consider the bike as a 2D graph with the origin (0,0) at the bottom braket. Measure your points of contact as coordinates. Use a plumbline and a tape measure.