I would say that in pretty much every way, the United MileagePlus program is superior to M&M. (I've had status with both, but have never made it to SEN on M&M.) As a foreign-based elite w/ UA, you're also exempt from the PQD requirement, which is a sweet benefit.
The only way I'd consider switching to M&M is if you think you can reach HON status, which is unobtainable for pretty much anyone.
Keep in mind, you can use UA GPUs on LH, and UA *G gets you access to the SEN lounges throughout LH/LX's network.