Well 3m's adhesive is the only one that I have used. I'm sure there are other manufactures out there that make a good product but I would still stick with 3m's 8115. And you are right it is expensive. Plus, you need their special gun to use it.
If you do decide to go with 3m there will be only two mixing tips that come in the package. I would highly recommend buying a few more. Once you use one tip and let it sit for 30min or so it's junk, meanwhile you may still have 3/4 of the product left. Just put on a new tip and your good to go. When you are starting a new tube you have to level both sides. To do this simply use the gun and squeeze out the product without a mixing tip. If you don't do this then you will get improper mixed products that mostly likely never harden.
I hope this helps. If you can't tell I love this stuff. In the collision repair field it is a godsend. It's saves a ton of time and it meets auto manufacture standards to replace welds in many areas of the vehicle.