Round. I think Anschutz thinks so too. They went to the 2013 flat action and that became their staple. Then they discontinued it and went to the 1913, and that's their staple. I've had a 2013 and I believe the round bottom 54 is better.
From my limited reading the flat got a rep of being hard to bed and issues
with the four beeding screws.
But some think they have it figured out with a plate bolted to the bottom
that lets them change bbls without disturbing the bedding.
I think Steve B. is one of the key players at this.
I shoot rounds but I'm just a duffer and they are cheaper.
I know torque plays with a round bottom in the larger calibers - to what
effect it has on a rimfire someone smart should have a better idea.