Hello Brent,
Purely for Hauptwerk purposes, I would say that the relevant differences are as follows:
UFX+:
- 12x analogue audio outputs.
- 2x ADAT outputs, giving a potential total of 28x audio outputs (including the analogue outputs and the ADAT channels). (It also has MADI potentially allowing further audio outputs, albeit at significant cost.)
- Thunderbolt + USB 3.0.
- Introduced this year (2017), so maximum useful lifespan.
- Review: http://www.soundonsound.com/reviews/rme-fireface-ufx-plus
UCX:
- 8x analogue audio outputs.
- 1x ADAT output, giving a potential total of 16x audio outputs (including the analogue outputs and the ADAT channels).
- FireWire 400 + USB 2.0.
- Introduced in 2012, but still a current model.
- Review: http://www.soundonsound.com/reviews/rme-fireface-ucx