Just stick 379s on it and be done with it. The interior ambience in first and standard is just as good as that found on a 222 and I'm sure they can be modified for 110mph without too much difficulty. About the only thing "wrong" with them is the doors aren't at the end but that's always something that runs around in some peoples heads as being a bigger issue than it really is. Heck you can even sell them as being around five years younger than a 222 whilst the 360s are a few years older (and would require a proper refurbishment to avoid a tremendous downgrade in quality).
379's should probably work fine.
the maintainance crews are already basically fully trained up( bedpan was 378's for a good while, I doubt there is much significant change in the running gear)
360's means a full training course and probably a few problems with reliability during the learning curve.
anyway, whatever they decide, an 2*4 car configuration is still going to be a vast improvement in seating capacity over the existing set up.
depends on leasing costs obviously, but 379 also has the advantage of end gangways.
if abellio get the 360's massively cheaper,I still wouldn't complain too much, as long as they are on time,all the time, and there's somewhere to park my backside.