I own the 50 watt MKI Rockerverb-head. I think the main differences between the 50/100 are the powertubes. Mine has four 6V6's. The 100 wattt MkI has EL34's. The MKII 50 has EL34's too.
Personally I favour the 6V6 Rockerverb, which to my ears sounds a bit more lush and creamy then the EL34 version. A tad more classic too. I can nail SRV: first gain stage of the drivechannel, mastervolume at noon. Mine is quite agressive in the uppermids, but the amp balances quite nice with the mastervolume past noon, bringing up the lowend dramatically. It's quite loud then.
That's why I use another tubeamp for rehearsels and small gigs.