When I was looking at the Mark V, I was also considering buying another Mark IV instead as I stupidly sold my MIV Widebody combo 6 years ago and regretted it ever since. What swayed me to the Mark V was the much improved second channel and second master volume/Solo boost feature and the additional channel voicings on the V. Not to mention the 5 year warranty.
All that being said, I play in Mark IV mode most of the time on this amp.