According to Macworld testing of mid-2013 and early 2014 MacBook Air 11" and 13" laptops, it would appear that 128GB drives are generally capable of 3xx MB/s to 4xx MB/s sequential write speeds and 6xx to 7xx MB/s for read speeds, whereas the 256GB model is capable of 5xx to 6xx MB/s writes and 6xx to 7xx MB/s reads. The numbers are from Blackmagic Disk Speed Test, which presumably is smart enough to test with a higher queue depth.
According to AnandTech, the 256GB model has 3xx MB/s for both the sequential writes and reads with a queue depth of 1, and 700MB/s for write and 800MB/s for read with a queue depth of 32. AnandTech is also providing information about random writes/reads, but only for the 256GB model, so, it cannot be compared with the 128GB one.
According to other websites that have tested the 128GB and 512GB models, the 512GB doesn't go above 7xx MB/s for either reads or writes, either.
So, if you're interested only in performance alone, then 256GB would appear to provide an extra kick compared to 128GB, but 512GB would hardly make a difference to 256GB (at least as far as sequential reads at an unknown queue depth are concerned). THIS IS ONLY BASED ON THIS INCOMPLETE TESTING DATA AND SEQUENTIAL WRITES/READS AT UNSPECIFIED QUEUE DEPTH. Random writes/reads are what would matter for swapping, and comparison test data is missing for such metric.
In other words:
sequential writes (with XXX queue depth) on 128GB is about 1½× slower compared to 256GB and 512GB model (and the difference between 256GB and 512GB is negligible)
sequential reads (with XXX queue depth) are effectively the same between 128GB, 256GB and 512GB drives -- from 6xx to 7xx MB/s
sequential writes (with queue depth of 1) are XXX
sequential reads (with queue depth of 1) are XXX
sequential writes (with queue depth of XX) are XXX
sequential reads (with queue depth of XX) are XXX
random writes are XXX; model XXX is XXX faster than XXX
- random reads are XXX; model XXX is XXX slower than XXX