I've been test driving a Tom Bihn Synik prototype for about a month.  Last week Bihn sent me the final production version of the larger 30L and the smaller 22L bags.  My wife and I gave them a test run on a weekend trip to NYC, and on to Florida for (her) work trip and (my) mini-vacation.

First thing to know is that there is a WIDE gap in capacity between the two bags.  I compare it to a Goruck GR1 vs. Echo:  the Synik 30L is more of a rectangular, traditional backpack shape whereas 22L (when fully packed out) takes more of a cubelike appearance.

On my 5'4" wife, the 30L was way too big on her.  The 22L fit really well.  For me, at a slim 5'8" the 30L feels side on my back, whereas the 22L sits a little high.  

My biggest takeaway with both models is the width vs. a Synapse - they just feel wider across your back.  My assumption is this is due to the change in the framesheet, as well as the ability to simply pack the bag to max capacity: the clamshell allows you to overpack the thing for hoarders like me.

Both have the same features and most impressively, the 22L can still take just as big of a water bottle with only a small feeling reduction in the capacity of the exterior pockets. 

I did a quick test pack, and the 19L synapse fit about a pair of shoes less than the 30L.  I had to be more creative in packing but the smaller bag still swallows up anything you pack.  To me, personally, I'd rather underpack the 30L than have the 22L bursting at the seams.  The bag just feels better when it isn't at capacity - you don't have to fight the luggage passthrough, the bottom exterior pocket is easier to fit out with my dopp kit.  My laptop slides in and out without having to wrangle it.

If you're tall enough to support it, go 30L. 

If you're under 5'7" or so, the 22L bag is still a noticeable and appreciated upgrade to Synapse.

Quick Gallery - Size Comparison