I doubt they affect flow either way to any significant degree vs tubing.
Parallel/Serial?
With two cards, I'd go with either and usually do stick to serial out of convenience fwiw.
It won't make much difference.
With more than two cards, I'd personally go with parallel flow and personally saw great results with that (very even temps). You or may not have to pay a little more attention to making sure that air is properly out of the blocks though.