It turns out that both spawner setups would likely perform the same
(Note: I'm assuming here that the spawn radius is 4 blocks away from the spawner [That's how the chicken spawner is in Skyblock at least I think]; I'm also assuming that 2 spawners with their ranges intersecting means double spawn rate in those spots (which should be true, but it's worth mentioning just in case). Also, I forgot to put slabs on top of the spawners in the second picture :P)
Key
Green = double spawn rate (where the spawn ranges intersect)
Yellow & Blue = single spawn rate (where the ranges do not intersect; blue for the right spawner, yellow for the left)
Total green blocks * 2 (because of the double spawn rate):
144
Total yellow blocks:
9
Total light blue blocks:
9
Total:
162
Final total (subtract the 2 spawners from the range): 160
Key
Here, the spawners’ ranges don’t intersect, so Yellow and Blue represent single spawn ranges instead of double.
Total yellow blocks:
81
Total light blue blocks:
81
Total:
162
Final total (subtract the 2 spawners):
160
Since the total potential blocks that can spawn mobs (I’m not sure what to call it) for both spawner setups is 160, then they should both should perform just about the same