I would definitely go for the Blizzard, for 2 reasons:
1. SCSI. It's WAY faster/more stable than the A1200's onboard IDE. Plus you can add more stuff (CD-ROM, multiple harddrives etc. - you need to fill up that tower! :-) )
2. From my experience, a 25MHz 68040 has a very little speed advantage compared to a 50MHz 68030. Of course it depends on what you use it for, but if you're no hardcore speed-freak, the Blizzard is the way to go in my opinion.
