First.- I don't think that the real factor of importance is the speed, with a 20MHz crystal you can make links in 12000 bauds with serin/out, that's because all the times are calculated for a crystal of 4MHz, for me the real deference between them are the space in the memory, serin/out take more memory than hserin/out.

The other thing is that with Hserin/out you can receive parity bits and multiple stop bits, with serin/out you only can receive one stop bit and no parity.