It's hard to say.
If both were in like-new condition, the old frame would be better.
Rust never sleeps though--so to a degree, it depends partly on how old the frame is.
I have seen a few pre-WW-II bicycles that I thought looked cool, and I thought about buying them and hanging them on the wall. I would not really ride them however, either with an engine or without--and if you ask on road-bicycling forums, you will hear the same things.
1) parts can be VERY TOUGH to find for these bikes, the wheels/bearings/tires/bottom bracket are not the same as modern versions,,, and
2) they have been rusting away (inside and out!) for 50-75 or more years. You cannot fix, or even see all the rust damage they may have.
People do have bicycles that old, that occasionally do get GENTLY ridden. But they don't get on the things and crank as hard as possible, or do anything to stress them hard, because they know what will happen.