I'm lookin' at the 16C54C Data Sheet and it looks like it's a 12-bit core with very limited resources compared to the '628A... Hardly worth the 30 cents savings if you were buying them...
Heck, if you've got a bunch of them on hand, you might as well use them...
BTW, I agree with Nigel... The 1% tolerance 4-MHz and 8-MHz INTOSC in many PICs provides very reliable serial communications as long as you avoid severe temperature extremes... And using special 1.8432, 3.6864, 7.3728, 9.2160, 11.0592, or 18.432-MHz "UART" crystals seems almost silly when you look at the bit rate errors for standard crystal frequencies -- even more so when using newer PICs with 16-bit SPBRG capability...
Have fun... Regards, Mike