I assume you are test-riding a new bike with an uncut ridiculously long steer tube.

So, if you think of the bike as having 3 points of contact, seat, pedals, and bars. Then you should be able to adjust both bikes to almost the same fit, to give a reasonable test ride. And, if you're ready to plunk down a few thousand on a new bike, the shop should be willing to help you with the fit on the demo (assuming you're not just yanking their chain).

Some of the angles will be slightly different so the seat forward/back position may appear different, but you should still be able to set them to almost the same position.

Assuming a casual rider, then I'd go with the 61, or even larger.

If you're doing competitive racing, then you might consider the smaller frame which would allow more lower handlebar adjustment.