Same predicament here... Two small factors that separate the two further more in my book are- 1. The selectable front LED indicator (power, limit, off) on the QSC and 2. The casters on the QSC being on the sub as to JBL's cover/bag with casters.
Keep in mind I've been a JBL guy and love my SRX but the previous PRX500 rig I had left a sour taste in my mouth. I've also personally heard the PRX618S-XLF and wasn't to impressed (then again it wasn't an ideal listening being that it was in a local "DJ" store, with some burned CD audio source, no tuning, etc.)
Just my two cents and FYI I'm leaning towards the KW's
Mike Pyle also gave a good review not to long ago of the KW181's if you search it.