From my understanding, the so-net cards do not have any daily limit caps, so you could use your entire allotment of data (usually 2GB) in a single day if you wanted to, whereas the umobile cards throttle you down to 128kbps after 100MB/day of usage.

The so-net cards have been sold out more or less since the most recent lightning sale last month, so I instead ordered the umobile card for an upcoming trip for $24 on eBay. I'll do a speed test and report back.