I've had 3 big trips this year (May 2018-April 2019) and all 3 were splits. First was B6 (positioning to West Coast), AA (West Coast to Europe in J), BA intraEurope & VS PE back to JFK. Second was AS (JFK-PDX & NH (PDX-SFO-TPE, TPE-PVG, HKG-HND, NRT-ICN, ICN-JFK) and the third will be UA (to West Coast, Excursionist perk flight in EU, Additional miles flight in EU & EU-JFK), AA (West Coast to Europe in J), BA (intraEurope).

The second trip to Asia was mostly via NH and the greatest return on my points in value but having some points in different programs increases flexibility and routing opportunities.

If it's between 2 1-ways on UA or a split between UA & AA it's about best schedule and reduced potential fees for changes and/or preferred seating. In your case, it's a balance between not having to potentially pay that $175 in additional fees or having miles in a different program that could be used for positioning or a different 1-way for a future award trip with restricted space. From what you've said..burn the AA miles.