I tihnk the 150's should be the 1st to be withdrawn then all of the pacers.
Pacers are ribbish, all be it rather comical.
I think a sentiment from the heart rather than the head that one! 150's are very useful on busy routes due to the door arrangement and more standing room - as the pacers can be. I know I will be destroyed for saying this, but I would rather be on a 3-car 144 or 2-car 150 on a very busy service than a 2-car 158 - despite my love of the 158.
Let me explain my reasoning.
If you manage to get a seat at a busy time but are not travelling very far, one has to fight one's way from the seat to the door on a 158, whilst on the pre mentioned trains, it is much easier to get off when it is your time. It is also much better for standing passengers as there is loads more room in the doorway.
Stick two or three pacers together for a busy train and you would find they do an excellent job. Hence I would be sad to see the 144 move to Manchester as they can often be found ploughing my regular route out of Leeds.