I've done both for our museum with tanks that are not available. Around here the problem is the cost of getting a tank boiled. Only one radiator shop left that boils, and he has a line at the door. Last time I called the price was approaching one Ben Franklin!
The POR tank sealer is a good product, but you must follow their instructions to the letter, or the stuff won't stick and it's tough to get out. Best results without getting a tank boiled have been with the following: Put a handful of hex nuts that are 1/2" or bigger in the tank. Used ones if possible, the finish will be stripped from them. Then mix the Marine Clean with hot water and pour in the tank. Rock the tank in order to knock loose the junk letting the nuts do the abrasive work, mainly on the bottom of the tank. Rinse and dump out the nuts. At this point, diluted muriatic acid can be used, but it must immediately be neutralized with baking soda. The acid will remove the rust, but it will also open up any pinhole leaks. Unless the rust is heavy, I avoid the acid. Then treat with the metal ready and apply the tank sealer per the instructions. It's very important for the tank to be dry before sealing. One way the speed up the drying is to connect the exhaust of a shop vac, but be sure the vac is clean, you don't want to blow in dirty air.
All in all, it is a lot of work and costs some bucks to clean a tank. I opt for a new one when possible, it's a lot easier and not near the headache.