The snap method works really well. I've always used pieces of plastic - HIPS, Sintra, ABS or even thermoplastics like Worbla to make my snap pieces that attach to the armor. I never thought about the webbing! I agree though, the webbing needs some extra care the heavier your armor is. And make sure to seal your ends so it doesn't fray!
I buy my Line 24 heavy duty snaps from Tandy. They have packages of various sizes and, depending on what options you have and how many you need, might be more cost effective.
For an undersuit that you get a long use out of, I highly recommend reinforcing your fabric where the snap will be inserted. This is especially true if your suit has any stretch to it. Make sure to get the correct iron on interfacing for the fabric you are working with. I just use an inch square piece to the inside of the suit and insert the snap through that.
You may also find that your snaps aren't closing at all or are very difficult. In my experience this is generally because when using the heavy duty snaps you need to have enough fabric on the post or too much will be left to fold over causing the snap to not close. This is also easily resolvable by adding in a little spacer, like felt for example, to give more bulk.
In the end, just treat the snaps a little delicately. Don't yank them apart, but rather sort of pry them apart and you'll get all kinds of life out of them. More than one snap definitely for larger pieces builds in some redundancy that helps should one of your snaps on either side come loose. As well, keep some heavy duty stick on velcro in your kit to do quick repairs until you can properly re-glue or re-set the snap at home.
Similarly this can be done with magnets, but that does require some hand sewing to sew little pockets to your undersuit to hold them. Otherwise it's a very similar process for marking etc.