I personally wouldnt use anything but OpenBSD. Just personal preference, there may be and there may not be technical reasons behind it. 😉
I managed to comfortably fit OpenBSD (without x since this was a firewall) on a 245.35MB hard drive. I think it even had the src on there.. (got the hard drive sitting here unused right now 😛). Ive also read articles on slimming it down to fit a 128MB flash card. Ive never tried the emBSD, but I also know many of the developers (well Theo anyhow) doesnt really care about embedded type work for OpenBSD.