I love both my Mark III and Mark IV, but if the best tone is what you're after, your option is the Mark III. Yes, the IV sounds awesome. Yes, it can get close to a C+, but the Mark III is clearer, more responsive, and has a rawness to it that makes it feel as if a blanket has been lifted off of the speakers. It's that noticeable. By comparison, the Mark IV sounds darker, and while it sounds amazing, the Mark III just sounds better, IMHO.
The biggest difference between the two comes down to convenience. The Mark IV has more to offer channel wise by giving you 2 channels with completely different tone controls, and while some of the controls are shared, you can get 3 separate, usable tones. With the Mark III, everything is shared, so it becomes a little more dicey. I'd highly recommend the R2 Volume mod to give Rhythm 2 a little more separation, but otherwise you can use the cleans that are dialed in, or you can use your volume knob to knock some of the hair off the tone.
For me, when I hit the studio, I bring the Mark III. If I'm playing live, I usually bring the Mark IV. For those of us that are used to JCM800s and working with your volume control to get your cleans, you'll be right at home with a Mark III. For those of us that are used to Diezels and multiple channel amps, I think the Mark IV will fit the bill.
I'll also second what was suggested above. Don't get too hooked on the color of the Stripe, because all Mark IIIs are amazing amps. The Blue Stripe is the most aggressive, but it's got so much gain, you'll likely never need it.