I'll weigh in here. I'm on an IP-256 and I am 5' 10.5". I completely understand you in-between-sizes-struggle. I have it with clothes, shoes, and bikes.
My inseam (measured the correct way, not the pants measurement) is 32.25". I'm a pretty firm believer that standover isn't the be-all and end-all of bike fit, but it's how most shops try to fit you. Actually, most shops try to fit you to what they have in stock, but that's another rant for another day. Anyway, I have maybe 1" of clearance in the stand over department, just for reference. Perhaps a bit more when the boys are picked up by the lycra.
Either way, I think effective top tube length is a far more important measurement for "fit", for me at least. If the ETT is too short, you will be very upright and often, the handlebars and front tire can/will interfere with your feet/knees. Sometimes your elbows get into your leg stroke area. This is OK on a road bike, but can be annoying or worse on an MTB where range of motion is very important. Your center of gravity is high too.
With a longer ETT, you will lean over more. This can result in lower back pain, especially if your core isn't strong. It can also cause arm fatigue as more weight is on your arms. You have less control over your bike as things are over extended or at their limits.
Sure, you can "fix" ETT with a stem length change, but I really don't like to do this too much. I think longer stems result in a bike that handles somewhat slowly. It also doesn't cure the problem of the feet and knees conflicting with the handlebar and front tire on turns. I would not really venture out of the 80-100mm stem range unless You have zero other options (you're REALLY tall or you already own a bike that is too small).
So how do you choose? That's rough. First I used the competitive cyclist website to take my measurements and make some recommendations.
Then I identified what type of rider I am and what type riding I wanted to do. I'm a faster rider who rides smooth, quick, non-technical cross country. I want to do more endurance XC racing. I don't mind being in a bit more of a race-oriented (stretched out) position.
Lastly, called on experience. I bought a Giant Anthem X2 in large. I rode it for 1.5 years. I learned that bike like the back of my hand. I know how it fits me and what I would change about it. In my case, it was just a bit too long and I often felt just too stretched out. Another problem I had was that since I had to reach further, when I stood up to climb, I was positioned further forward with unweight the back tire and caused loss of traction. (long chainstay length on the Anthem didn't help this either.).
Building on the "based on experience" approach mentioned above, another tool I used was to gather the geometry of all the bikes I have rode for some amount of distance in recent memory. I then went to each manufacturer's site and dumped the geometry. This can be tricky as not all manufacturers list all dimensions or call them the same names or report them in the same units. But the main ones are usually there is you parse through them.
My list had the following bikes:
Giant Anthem X2 (Large)
Specialized Camber Comp (Large)
Niner Air 9 RDO (Medium)
Then I listed the frames I was considering:
IP-057 (Medium and Large)
IP-256 (Medium and Large)
Lastly, I included the Scott Scale because I heard it had identical geometry to the IP-256. Which it does!
Here is the chart I made:
Here is the dimension key:
Armed with this information, I decided that the IP-256 in 19" would be a good fit. It's ETT of 620mm was shorter than the Giant's 635mm. That's good as I wanted to be a bit less stretched. I knew I could also go with an 80mm stem on the IP-256 compared to the 95mm one on the Giant. This would buy me another 15mm less stretch. Cool. However, I rode the Air 9 RDO in medium and it felt cramped at 607mm with an 80mm stem. After that, I checked some of the other measurements to make sure nothing was goofy. The chainstay length (my other complaint about the Giant) was very reasonable and a good inch shorter on the IP-256/IP-57 which would make for a better climber and faster turning bike.
My point is not to bore you with how I found the fit on MY bike, but it was to show you the method I used to choose when I was in your exact position... Medium or Large for a guy on the fence.
If you don't have own other bikes, it might be worth it to borrow or rent one or two. Demo days also rock for data gathering. It's a bit of work, but that's the price you pay to order a Chiner sight unseen.
