I actually found that a low dose of BB can give my running a boost, particularly if I have been experiencing symptoms whilst running. With the bad heart beats gone, I can feel better and run better. I will say though that the BB can also work the opposite direction if I take more than I need.
If you are speaking of the Atenolol, a beta blocker, I claim it has no effect on stroke mitigation, other than to the extent that it stops any episodes of AFib. From what you say your AFib isn't causing you athletic activities any problems. That suggests to me that includes you resting HR is reasonable as well as your exercise HR. All this being the case and if your BP is also in a good range I'd say I'd be inclined not to take the BB. I take a BB and believe it contributes to my generally lower energy level, to a general low level feeling of fatigue. This doesn't stop me from exercise, but as I am in permanent AFib, I am more limited in the intensity level of exercise.
My thinking is give the new medication a try, it may not have any noticeable
side-effects for you. When I was in NSR (and up to the age of 67) I "ran" (10+ minute miles for 3 miles or thereabouts). The BB didn't seem to have much effect on me, none that I noticed.