All three are great knives. The reason I've never picked up a Grip, despite my admiration for the knife overall, is that I don't like the thickness of the handle for the size. Just too bulky for me. It's a classic, just not for me.
Joe,
If you have handled the mini-Grip, then you have made an informed decision and can forget the following. If not, what I say may of interest. I have a mini-Grip and find the handle a bit small for my grip. I still use it, however, and think it's a great knife. My point is that if the mini-Grip handle was thinner, I wouldn't like it. Because of the smallness of the handle in general, I believe the added thickness is quite necessary to give a decent grip to a small handle.
YACOUB: As for the other knives, I don't like usually like all metal handles, especially steel (the ATR). Others have already pointed out the reasons. I have a few with aluminum handles but aluminum warms faster in cold weather and they are power-coated or have non-slip inserts for a good grip.
The ATR is not a small knife. Its closed length is 5 1/8" which is only 3/8" less than the Military, which is considered a large folder. It can still be EDC'ed but just want you to know it's not small. The closed length of the Stretch is 4 1/4". A big difference from the ATR. To compare the ATR with the mini-Grip as a self defense knife is a mistake. The ATR is quite large and was designed as a tactical knife. The min-Grip is more like a "pocket knife" compared to the ATR. Sort of like comparing a 9mm semi-auto to a 22 long rifle revolver.
I can see comparing the two Spyderco's but the mini-Grip needs to be assessed on its own or against other knives of the same size and similar design. It really doesn't fit into the Spyderco group as fair comparison.
The Stretch is a fine knife. The Kraton inserts provide a good grip and the knife is comfortable to carry. From what you have said, it would be my choice of the three. Some say it's a little thin but that also helps may it comforable to EDC. You must make the final choice but I believe you would like this knife the best of the three.
Are there other to choose from? Many. But that's another story.
Regards