Pros & Cons are in the eyes of the beholders.

Other differences:
- fuel tank location (in-cab or under-bed)
- left-side lower-cowl open/close air-vent vs vent windows
- wider seat in the 51

As crenwelge posted: " I would use the one with the least rust. "
I'd pick the one with the less cab rust. Check carefully around the interior cab floor and the lower rear cab corners (poke with screwdriver).