I think people misquoted the "8 GB" issue. Quick google reveals that the bug is a "8 MB" issue. Drives that experience unexpected power loss sometimes report their size as 8 MB on reboot.
http://news.softpedia.com/news/Intel-320-Series-SSDs-Affected-by-Firmware-Bug-that-Causes-Data-Loss-210809.shtml
This seems like an edge case bug that is unlikely to cause issues for the majority of users (but still sucks if it hits you).
From a reliability stand point, I'd also agree that based on review/personal experience the 320 seems more "reliable", but that's just a gut feeling. The 520 could be just as reliable than the 320 if not more so - it just hasn't been around as long so it's hard to get a "feel" for it.
For most typical consumer "real world" use, there will be very little noticable in performance between the 320 and the 520. Random access latency on both is comparable, which has the largest impact on consumer performance.
Not sure what exactly you're asking about IOPS, so I'll answer it as fully as possible:
IOPS is the measure of Input/Ouput Operations per Second. More/second is better in terms of performance. The chart you linked seems to show the IOPS over an 8 GB span vs the full drive - this shows how the drive handles reads and writes to a small portion of the drive vs the full drive. You'll see a bigger discrepency across a mechanical HDD since random operations will be faster on the inside of the platter than the outside (due to rotational latency, primarily). For SSDs, the span shows how well the SSD is able to balance reads/writes across the drive for best performance - ideally there should be no difference between a full drive span and an 8 GB span. Since the 8GB issue was misquoted, the results you linked don't really show anything.
As far as a better "read drive", IOPS are an artificial benchmark and should not be used to extrapolated determine the overall performance characteristics of a drive without considering other benchmarks. Yes, the 320 is faster for random IO, but unless all you plan on doing with your drive is 512 byte random reads/writes all day, the 520 will likely be faster since it handles large writes/reads faster due to it's SATA III interface and SandForce controller's data compression/deduplication.
Long story short - 320 "seems" more reliable, while the 520 is faster (in most benchmarks/real world use).