I have owned neither but I had a Mark IV for a few years and currently use a Roadster so same-ish ball park.
The RK will sound way bigger and saggier in the low end where the Mark will be super tight and clear. The trade off (at least to my ears) is that the Mark sounds thin and nasally when compared alone but they cut the mix better and have that signature sound that really kicks butt.
The RK/Roadster can get pretty close to Mark territory if dialed in right but they really shine as metal rhythm machines. The added low end frequencies, subdued mids, and gain texture are perfect for chunky palm muted metal but good if not great lead tones are also there on channel three that remind me that I'm still playing a Boogie.