PixelPaul

See size chart below. I'm 5'6", which according to the chart puts me on a XS. But I have a 31.75" inseam, which puts me on a M. Should I split the difference and go with the S? Not sure what other considerations to account for?

Nickolassc

You can probably make both fit. I personally prefer to go smaller and have more seat-post stick out, raise the bars, and longer stem in these situations. A bike that is too big can never be made to fit.

You have long legs and a short torso which generally means sizing down, also, measure your arm length, a rough rule of thumb is that your middle hand (higher palm crease) to your shoulder socket should match the length of effective top tube plus stem length. So if you get the XS with 520 mm top tube and you use a 100 mm stem, that puts that length at 620 mm from saddle to bar. How does that compare to your arm length?

Kontact 

Your inseam is somewhat long for your height, indicating that you could have limited reach. But it is much more likely that your reach and handlebar height will be most appropriate to the S. So unless you have an issue with back flexibility, the S with a 90 or 100 stem should work well enough.

The M is bananas too large. But I don't think their inseam/height ratios are average.

Iride01 

Have you stopped growing? If so either bike can probably be made to work. If not, I might consider the larger one more closely.

It still boils down to what feels good to you. Is the shop giving you time to ride them with a basic fit on each?