I almost passed on commenting on this, because it usually is mostly a case of various owners of the bikes in question providing justification for their own purchase. Then I read some of the comments and I was impressed. Maybe it is because this group of TW owners understands the big difference between street bikes and dirt bikes.
First of all, forget the V-stroms. Horrible off-road, although good street bikes. The DRZ400S is not a bad choice, and two of your friends already own them, so checking one out should be easy. A well set up DRZ is a better dirt bike than either the KLR or the DR. Slightly better than a DR, which is in turn better than either generation of KLR. The older KLR is a better dirt bike than the newer one, since it is much lighter, while as a pure street bike the new KLR is the best budget tourer made. As some here have stated, it has a fairing and substantial wind-screen. It also has a huge gas tank, giving an effective range of well over 250 miles. You could ride comfortably with your friend who owns the Vulcan.
The new KLR would be my last choice in the dirt, however. At 430 pounds with a full tank, it is 110 pounds heavier than a DRZ, and taller and more top-heavy to boot. Not to mention all of that pretty plastic to smash if you ever go down. A good dirt bike it aint, IMHO. I'm not familiar with the terrain on the T.A.T. , so you are the best judge of whether or not you could put up with it in the dirt.
If you are tall(over 6 feet)the older KLR is not a bad dirt bike. Not much heavier than a DR, but the DR has one huge advantage for average height and shorter riders. The DR is designed to have a choice of suspension settings for two seat heights. The seat height on an older KLR is listed at 37", while a DR has a choice of 35" or 33" depending on setup. In rough going off road, the ability to get a foot down has saved my butt many times. I'm 5'10" and I can flat-foot the DR on the low setting. It still has almost 9" of suspension travel on the low setting.
If you are average size you will likely need new springs on all except the new KLR. Both the older KLR and the DR have bad seats, and the DR needs a larger gas tank, unlike the KLR. All of the bikes I mentioned have a 5 speed gearbox and that is one less gear than they need.
Frankly, the DR is a very capable street bike with just a few modifications, and I personally find it to be much better in the dirt. Your experience may differ.