A 508 has a lot more history than 507. They started off as four cars, worked in SWT land, gifted a car to 455s, and then separate ones - but same class - worked for Silverlink and SE.
From a local perspective, arguably a 507 has more historical value. These have spent their entire lives on Merseyside and were new for this railway, will have a longer career than the 508s and are said to be in better condition overall.
It would be fabulous to see any of them saved, even a single carriage or failing that a cut cab section and I would put my hand in my pocket to make it happen. There are however a few reasons to go for 507001: first delivered and in traffic, carried VIPs including Queen Elizabeth II on the opening special, likely to be one of the last overhauled and kept until the end barring a 507019-style disaster.
Going for 508111 would be a fool's errand, because it's so tatty that you'd have to re-apply the wrap, an expensive undertaking. I don't understand the desire to preserve it only because it happens to have a Beatles-themed wrap anyway. Having worn that wrap since 2011, I also doubt its bodywork is in great condition.