Hardware RAID often works best with identical disks, but the Linux software RAID is actually very forgiving, so I think the slight differences will be OK.

I am hoping that the striping will give me more bandwidth. Apparently there is an advantage to using an extra parallel swap partition on your sd card if the sd card is fast enough, so I was hoping that I could get the same advantage to disk access in general.

I made a large file from /dev/urandom and timed when copying it to both the MMC and the sd card (from root). The sd card and the MMC seem to have roughly the same speed, at least for sequential writes.

Does anyone have any experience with doing this? Are there any problems? Any advice?