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! 
I agree with everything Oldsmobile_Mike said, and would like to remark that for the ACA timing fixes and the lack of FPU I will not hesitate to choose the DKB Cobra.
If you want to reach ACA performance, just simply overclock the card a bit (the Cobra is very friendly to overclock and so are 68030 cpus).
