I used the full POR-15 solution and am happy with the results, but cannot compare to the KREME solution since I haven't used it.
Everything went smoothly on the POR-15 except the putting of the actual coating on. The company recommended moving it around over a long period of time, but I'm a little impatient, so I missed a spot or two, which were really hard to see without a small mirror. Here are some hints:
1)From previous experience I would recommend thinning the coating out and doing multiple thin coats. This will take longer, but will also have a more even coating. You will want to rotate the tank allot then drain, let dry then repeat. The initial coating wont be as thick, but it will dry faster and with multiple coatings you can get the same thickness, only more uniform. This is assuming you don't already have any leaks, and your just trying to prevent one from starting. One acquaintence did not thin the coating and got a satisfactory result. The trick is to keeep rotating the tank to obtain complete coverage.
2)The coating has Methyl Ethyl Ketone (a known carcinogen - very nasty stuff) so do it outside.
3)I also had very good luck using about a cup of 00 buckshot in the tank to break loose the scale before the acid treatment. Just put it in, close the cap and shake it hard for a few minutes. When you are done, shake the buckshot and debris out until there is nothing rattling around inside. You may be surprised by how much junk actually comes out. Be sure to count the number of BBs you put in and that you take out
I selected POR-15 for the concerns you list, but long-time owners of the model I own have used KREME w/o troubles. For me, the cost of POR-15 was just cheaper.