This is a bit of a speculative one here, but it suddenly occurred to me that I had forgotten all about the 7 class 155 Sprinters, so I looked them up (yes, those funny little units that were converted to the class 153s save for 7 of them - There originally were 42 of them).
Anyway, it says they were the first of the Sprinters, and did lots of testing for the other later types.
It got me thinking, if you were the person or one of the people at the NRM who decides these things, what type, out of all the Sprinter types (class 150, 153, 155, 156, 158 and 159), what would you choose for the national collection??
I would be inclined to say either a class 156 or '158, but each type has it's own merits and pitfalls, so it isn't as clear cut as all that.