I haven't tried the RP-350, but as owner of a V-Amp3, all Marshall-based models are pretty good.
Although I'm not an avid Marshall-y sound user, my personal favorites are:
1. Brit Classic 50W aka "Red Pig", based on the original Red 50W Plexi (also found on the V-Amp2)
2. Rock 59L, based on the 1959 Head, pure 100 Plexi goodness. (exclusive to the V-Amp3)
Using a DAW, use either of aforementioned V-Amp models with no cab, then use a IR loader and put it through an Ownhammer 4x12" Marshall 1960B w/25W Greenbacks and bingo! Pure, unadulterated vintage Plexi bliss.