I liked the BT-014 when I had them on my K6, but I have never had them on the R6 I have now, so I don't have a direct comparison. In terms of mileage I am expecting to get about 20,000 miles out of my BT-021, where I got about 5,000 out of the BT-014. I commute about 70 miles per day on the interstate these days, so I can get higher mileage than other folks can get.
In terms of grip, I would say that the 021 has slightly less than the 014 during extremely spirited riding. I used the 014s for a Novice trackday and they would probably work okay for an Intermediate trackday, but I don't know if I would use the 021 for an Intermediate trackday. For spirited road riding though, the 021 will hold up just fine.
While I don't know this for certain, I sort of suspect that the 021 uses the same compound for the sides as the 014 or 015, which is why it has such good traction. If it's not the same, it is similar. The only differences are the harder center for longer mileage and a slightly stiffer carcass, which also helps to increase longevity.
Overall I rate the 021 as close enough to the 014 in terms of grip, but beating the 014 into the ground and then spitting on it in terms of mileage.