The Cobra card will let you add an FPU, which can be clocked higher than the CPU, if you care about that sort of thing. Also it already includes an RTC, and optional SCSI. And it's less likely to require any of the "timing fixes" that some A1200's need with the ACA's. But the memory access speed on the ACA will most likely totally crush the speed on the DKB.
I've got an idea - try them both, and post benchmarks! :idea:
Oh - and welcome!
