I too faced a similar delemma several weeks ago. I was not able to lay hands on a Ibis Mojo, so I had to go seat of my pants and buy without a ride on one. I think there are several factors that should be considered, this knowledge was gained by personal trial and error, and comparing my new bike and the sensations/feedback give to me by the bike. There was also a bit of blind luck involved as well.
First you must have some sort of an idea on if you like a larger or smaller bike. If you do not know that just climb on some and decide what the lesser of the 2 evils is, larger or smaller? It does not matter what kind or brand, just ride some bikes that are of different size. Consider if you like to be errect, spread out, if you perfer to glide and ease a bike or to manually handle it, steer with the bars so to speak.
I am 6' with a 32-33" inch inseam, or sole to sack as you put it! Those measurements would pretty much say Large to most, or so I'm told, although there are some who say Medium?
I like a large bike, I have 2 other bikes that are XL, or 21.5" frames, I opted for an XL Mojo.
I am super glad that I did, the XL mojo is slightly smaller than my 21.5 Trek Liquid. When I received my Mojo it came with a 50mm stem (that was the blind luck part) although I am going to go with slightly longer stem for comfort purposes I could not believe the handling characteristics that come with the 50mm stem. The ease of lofting the front end with no notice. I have a 110mm stem on my other bike, let me tell you as streched out as I am on it, it is quite comfortable but if I suddenly happen across a situation that requires an instant leap, or huck in order to avoid a front end first plunge....to bad, I am stuck like chuck and just have to go with it. With a 50mm stem it seems as though I can be actually dropping off of something and still pull or pedal the front end up and drop off rear wheel first if that is what I want to do. Of course this could be the MOJO too?
Hopefully all of this makes a little bit of sense?
Really just consider what terrain you ride, how you ride, what you like, and take the plunge.
I think if I were you I would consider the Large!