Both cards have one major drawback so you have to consider each: (for OS4.1)
CSPPC - 128mb Ram, OS4.1 needs at least 96mb so once you run some app's that's it, you run out. Zorro Ram can only be used as Page File in OS4.1 so this is not a solution.
BPPC - SCSI-2 does not work with OS4.1 so you are stuck with slow IDE, it's a real killer of the performance for this OS. (n.b CSPPC UW SCSI does work with OS4.1)
For OS3.9 you will barely notice the difference between the cards, 128mb is enough, SCSI-2 is 10mb/sec vs UW-SCSI 30mb/sec still much better then IDE (2mb/sec)
Your main consideration should be how you want to expand your Amiga, if you stick with an A1200 wedge then the only possibility is BVision for RTG however when in Tower you have various PCI or Zorro II options. Note A1200 in Tower does not have Zorro III possibilities meaning in an A4000 you have more options for upgrading.
I hope this helps
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FWIW I personally belive the A4000 with CSPPC & GREX PCI is the ultimate setup followed by A1200 with GREX PCI. After that both will have similar results with Mediator PCI.
For Purity an A4000 in its original case with only Zorro Cards is your best bet.