aggiegrads asked "What's the rule of thumb" -- somewhat moot because you are a self-professed mutant and therefore you need a specific answer.

I recommend trying any of several online fit calculators to get an informed baseline that is semi-custom to you. competitvecyclist.com, et al. Yes, all the little details matter, blah blah. But within that, I strongly favor the top tube as the cardinal measurement. It has served me well in my situation, long legs, short upper body -- opposite problem you have where I often wind up feeling too stretched out unless I get a relatively short TT.

My hunch is that you'll be better served by the smaller frame, but ONLY IF you can get by with a 130-135 (are 140s out there, stable on that small a frame?) or so stem in place of the 110 or so it probably comes with. But that's just a hunch, no one can tell anything definitive from the vagaries posted to date. Except this: do NOT compensate for upper body length with seat setback. Seat setback positions your legs/butt/etc. (basically knee) relative to the crank, or your pedalling efficiency (+/- comfort) suffers. You adjust for your upper body with stem length within reason, and top tube length for the macro.