Well, you must be doing something wrong to get so much mileage from your Metzeler, HD2Rocket3! After all it is generally considered by some here and over on the .com site to be "a piece of garbage" and a "known safety hazard", even though it was specifically designed and purpose-built for Triumph and their Rocket III.
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Most of the the above is said tongue-in-cheek]
I recently returned to the Metzeler after a not
completely unsatisfactory experiment with a car tire on the rear of my 07' Standard. I chose to go with the OEM tire for two reasons; firstly, it is the OEM tire and secondly, and more importantly; others had indicated that while the Avon was good for some additional mileage vs the Metz, it was not as good in the rain and on wet surfaces as the Metzeler has proven to be. Of course, no MT will outperform any CT on the straight and level in the rain. Me... hell, I avoid riding in the rain at all costs.
Although I am a pretty aggressive rider, burnouts and wheelies are not in my repertoire. I do what I can to extend the lifespan of my tires by frequently checking for and insuring their proper inflation. I fully expect to get 8-10K miles on my new rear tire, which is about par for the course for a big bruiser cruiser such as ours.
The differences between the Marathon and the Cobra are incremental, I'd suspect, when you get right down to it. Not much diff in cost factor and somewhat a tradeoff between their performance and handling characteristics from what I've heard and seen. Hope that's helpful...