installing system on SD card?

I'm considering my next XBMC system. I want to run it on ubuntu or openELEC. The system should be as cool as possible, so I thought of an SSD (no media storage, only OS).
But SSDs are expensive. 64 SDXC cards are much cheaper.

Does it make sense to install a system on an SDXC card, and let it be the read/write system disk?
How does it compare speedwise to an SSD? (My guess is slower, since its not through SATA interface, but how does it compare to a rotating drive?)

Thanks,
Dan