Discussion as to whether a 319 is significantly worse to travel on than a 350 is, IMHO, not all that relevant as since when did passenger comfort and satisfaction really, honestly and truly, become a factor in deciding what stock gets shuffled where? Money and costs, thats what will be considered.
One post has highlighted a very important point - the type of lease the 319s and 350s are on - the latter usage/mileage based, the former seeming to be fixed fees. Such a difference could well have a significant bearing on decisions to deploy 319s vice 350s.
This could also be linked with what is the most effective (and cheaper!) option for the railway as a whole - expensive refurbishment of 319s to fit AC, improve performance, etc to allow them to keep up with 323s and/or meet the required Northern Express criteria OR just refresh the 319s and deploy them on the shortest LM turns (even just the 45 minute Tring services look as if they need 4 OP diagrams, possibly 6 at peak time) thus allowing some 350/2s to go to Northern with less expensive modifications needed. Yes, some LM services could be seen as being downgraded, but the compensation could be 12-car sets in place of 8-cars. Given the lease charges, it might even be possible that a 12-car 319 set comes in cheaper than an 8-car 350 - an issue that will play heavily on any decisions, I'd have thought.
One option I've not seen mentioned is just SOME of the 350/2s going to Northern - perhaps just enough to cover 323 turns to Hadfield and Stoke, along with Northern Express services with 319s used vice 323s on the Manchester-Crewe, Wilmslow, Airport, for which there seem to fewer performance related issues with 319s vice 323s. This would allow LM to retain 350/2s on the longer Northampton diagrams (thus placating a few posters here...)