Mark 4s all the way for me! Pendolinos are dark and claustrophobic. I've got to say, I'm quite happy with Pendos; now that the faults and niggles have largely been dealth with, I think they're a good quality train and quite pleasant to travel on. BUT, it's interesting how they stack up against the mark 4s: Both are tilting profile vehicles designed for 140mph operation and yet the mark 4s offer a much brighter, pleasanter environment, better window alignment and seem to have MUCH more space entirely. the 20 year old train wins hands down over the younger Pendolino. I do sometimes find the ride in a mark 4 to be a bit stiff though.
EDIT: And if East Coast is to be handed it's own Pendolino, I'd want to see it used on the planned London-Edinburgh daily "fast" service: If it's an attempt to compete with the airlines, then the airline passengers it attracts won't notice much of a difference! It's good to have a standard service level, isn't it?

Plus that keeps it on one regular diagram with a standard maintenance cycle, and means that there won't be a variation in interior standards across the regular services. Plus it would set the Fast service apart from the other slower anglo-scottish workings and help to differentiate it from the norm.