I would prefer the A, but there are still a lot of them available.
If the 420 has either live hydraulics, live PTO (?) or wide front end, it would be more useful than the 37 A, so I would expect it to hold its value better in the future.
The 420 is lighter and smaller than the A, making it easier for more collectors to haul to tractor shows. The 420 might also be a better choice to run a belly mower, making it more desirable as an "estate" tractor.