Is there any difference in ballistic performance between the two? What do you recommend?

Pigeon shooting
There would be no noticeable difference in ballistic performance between a 28 and a 30in barrel on a semi-auto, and therefore it is only a matter of personal preference.

Do remember, however, that an auto has a long receiver, so a 28 points and handles like a 30-inch break-action gun.

Bearing in mind that you will be shooting from a hide, or maybe roost shooting in woodland, my preference would be for the 28.

In fact, years ago when I had a 28-inch Remington 11-87, I bought a spare 26-inch barrel for shooting from pigeon hides and in woodland.

With the short barrel fitted, the gun handled like a 28-inch break action.