Even the 50 watt Marshall will be rediculously loud. I've asked the same question several times before at various places, and the common answere I got was... the 100 watter will have somewhat of a bigger feel to it and will have more headroom (which may or may not be a good thing), and the 50 watter will start to break up earlier (which may or may not be a good thing).

JFRICK has a JCM 800 (can't remember if it's the 100 watt or 50 watt version), so hopefully he'll chime in and tell you his experience with it.