I'm a Mark IV owner and recently acquired a Rivera TBR-1SL.
The Mark IV is a great amp, but does have its weaknesses. The clean channel is mediocre. I just don't like it. It's spongy, not very warm, and doesn't sound "organic." It can be made to break up slightly, but doing that doesnt sound as good as using a tubescreamer.
The middle of the road gain is pretty good. Nice n wide, lots of crunch. Ala AC DC.
It has some nice, middy, smooth, lead tones that I really like. But it can get muddy at high gain. Petrucci used the Mark IV exclusively (I think) on the G3 tour. The Mark IV is poor at high-distortion rythm guitar. It just turns to mush.
For what you described, a Mark IV might be slightly better because it's more versatile. Just don't get the combo like I did, you miss far too much bass response having an open-back cabinet. The Ecstasy will have a better clean channel, and probably a better lead channel, but it doesn't have the same level of flexability.