1. Really no difference in breathing I could discern.
2. M1 are "officially" rated for up to 80% O2 so can be used in various configurations from back gas to stage bottles. You absolutely cannot use a T2X beyond 40% O2.
The difference is in the 1st stage construction/materials between the various regs. If I had a do over, I would likely swap the T2X for M1's with Monel.
A basic brass reg can last a lifetime as long a PM is performed and you clean it properly so really no need to spend a lot of money. The cost of a T2X does not represent "better quality", it represents the technical difficulty in machining a piece of titanium alloy vs monel, stainless steel, or brass.
Which ever reg you purchase, consider the environmental seal kit on the 1st stage - Some Atomics come with it installed and some do not (M1 and T2x do)
The swivel is no big deal - a 2nd stage with a 7' primary hose or miflex hose does not require a swivel.
I transitioned from single tank/T2x -> doubles/side mount/M1's, -> CCR with bailout bottle and 40%+ Nitrox/M1's
I vote for M1 + Shearwater Predator SA vs T2x
Dwayne