Tom Bombadil

Given all of my test riding (have test ridden something like 80 bikes, of many types and sizes), I would estimate that your best size in a mountain bike / hybrid would be around 19". Although this can vary as not all 19" bikes have the same top tube or standover measurements. These hybrid frames are designed to fit a wide range, so their 20" would cover the 19"-21" range. So I think you would be closer to fitting their 20" frame than the 17.5". But this could come down to which one feels best to you. Some people like a bike that is a bit smaller.

Their 17.5" hybrid is roughly equivalent to a 52-54cm road bike size, which would also tend to be small for someone who is 5'11" w/32" inseam, unless you have shorter than normal arms for your height. With "normal" proportions, a person who is 5'11" will tend to fit a 57-58cm road bike.

I would think you would have to have a lot of seat post exposed to get your saddle into the right position on the 17.5" frame, given that your legs are 3"-3.5" longer than mine and I have my seat up a few inches on my 17.5".

If you had the seat adjusted to the correct height on both the 17.5" and 20", it would be the same distance from the ground. However the handlebars would be lower and closer on the 17.5". It is left to you to determine which feels best.