hey, bit of an essay here, but stay with me
i was deciding a year ago now about exectly this and couldn't get what was the difference either, so i bought the split dump pipe... and that was a wrong move. see, the pipe was not manufactored to a high level of acuracy and, as it's been mention here before, prevented the actuator flap to open fully which resulted to boost creeping. i had a t piece boost controller at that stage - which i blamed and replaced with electronic boost controller which evidently did not fix the problem. the boost would just keep on rising as the car goes through it's rev range and if i don't back off it, it would overboost to 14psi and more potentially. to fix the problem i had to remove the dump pipe and go at it with a grider, sand paper - anything i could find. in the end it all worked out, the boost is not creeping up anymore and the dump pipe is doing what it supposed to do, but if i was to do it again - i'd go for bell-mouth dump.
ps. another interesting thing with my split dump pipe, maybe other as well, is that it increased my boost from 5psi to 10psi (if the EBC is switched off) because of the small bypass pipe or the split dump pipe does not have enough room for the flap to open up fully.