It's a matter of personal taste. I prefer the tone I get on my Mark IVs and my Mark IIIs to what I have been able to dial in on Mark Vs. There are definitely times when I had the versatility of the Mark V, or th clean tones the Mark Vs achieve so effortlessly, but in the end, I feel at home on my lead channel.
To my ears, the Mark IVs lead channel sounds much less sterile than the Mark V's channel 3. I really wanted to have a Mesa that had a beautiful, chimey, bell like clean channel, and a fire-breathing lead channel, but the lead channel is missing the feel and the grit of the Mark IV.
I may be in the minority, but I think a really great product idea would be to create a modern, everything-but-the-kitchen-sink amp based on the Lonestar's clean channel, the 2nd channel to feature IIC+ designs (maybe 3 modes that feature different versions of just the IIC+ versions), and the final channel based on the insane and complex gain structures of the Mark III revisions (with modes that feature no stripe, red stripe and blue stripe). Now, that would be much simpler design wise because it only changes a few caps (typically) that can be switched in and out of the circuit based on the revision, and would give some different "colors" of the best amps for is high gain maniacs.
... Of and should feature the Coliseum power section featuring Simul-Class. I think that would be a powerful amp that would set a new standard for modern amps.
Sorry, to get off topic. So, I would go with a Mark IV. I think if you tried the lead channels on the IV and the V, you wouldn't be able to justify the lower price and inspiration from the IV. Weak ways, if possible; try both out and see what speaks to you, but the IV is more flexible on the lead channel, and that other channels feature shared controls...