MT21 vs. 606
Obviously, it all depends on how you use the bike. I'm not a particularly aggressive rider and managed to get 3900mi on my 690ER out of an MT21 rear. I'm thinkin' probably 40% dirt - forest roads/two track. I spooned on a 908 I had with some life left that spent most of its time on my SE. The paired MT21 front is now sitting at ~4400mi. I'm guessing it'll run another 600mi before it's shot and the salvaged 908, too. It's probably obvious that I run my tires all the way down, but that's yet to be a problem.
I know it's expensive, but I have to say I prefer the 908 (Rear) over the MT21. I've run the 908/606 (R/F) on my SE pretty much since the beginning and have gotten 3500mi/4500mi (R/F). The 908 I've now got on my 690 really works well, even though it's getting a bit thin. It's a great tire for the lighter bike.
A bit off topic, but I did make the mistake of putting a set Heidi Scouts on the SE. I found them to be fine for the road, but pretty miserable almost everywhere else, especially the front. I'm sure a more capable rider would have a better time, but that ain't me. I didn't even get the great mileage I was supposed to get. If memory serves, it was just under 4K for the set.
I've never run the T63, but the Mitas E07 or E09 might be something to throw into the mix. I've read good things about them. No personal experience, though.
Kind of rambling, sorry. My take is that the MT21 and the 606 are comparable tires for my riding and the difference in life is not significant. The MT21 is noisier on the highway. The price difference has been negligible.
Bob in Monterey