Hardware RAID5 will be orders of magnitude faster than software RAID5, especially in writes. Reads will be faster, but not by as much. Recovering a failed drive should be much less intensive under hardware.

Of course, all that assumes a solid RAID5 software implementation, which, from what I hear, simply does not exist in the Windows one. Every Windows admin I've ever met eschew it completely, including the rare ones who actually know what they're talking about.

You could go with an OS that has software RAID that's not failed on me in ten years or so, though: Solaris. It'll cost you $20 to download it.