OK experimenting has been done ... verdict is ...
OK no verdict really ... but there are some key similarities and differences.
OK the acid in kreem will eat aluminum, and it will eat anything in its path really, except steel is on its indigestion list ... so very very slow. Your tank has some questionable spots, do POR. If its solid generally but very rusty, muriatic acid is better (kreem acid is Muriatic acid, but I buy it off the street at Ho Depo or Ho's...
The POR acid will clean out the petcock and other sheite so you can really do it with them in there. And it needs 1 hour to etch it well, so might as well use it to clean out the petcock. muriatic acid will eat up the petcock in an hour, so use duct tape, but 5 mins is plenty ... max 10 mins so duct tape is plenty fine.
They both cause your skin to start burning pretty quick on contact so its a close game, but muriatic acid hurts more sooner.
The Tank has to be dry with POR but not Kreem. You use MEK to remove moisture a good bit. No MEK with POR, but a tank etched with Metal ready (the acid part of POR) has a chemically applied coat of zinc, so it rusts slower, as in overnight it will show spots of rust, but no biggie ... you can dry it with a hair dryer if you want it faster.
With Kreem - Muriatic acid will take it to steel and it will rust so fast you'd have to follow that with Hot water ~2-3 gallons with baking soda and slosh it and toss it and promptly dry with a hair dryer. Then MEK to get the rest of water out.
Here is the biggie then.
Kreem mixes with MEK, and so does POR - they dont say it does though, and it certainly doesn't smell like it ... but it does mix and thin out with MEK/acetone. I'd like to dilute the por where its very very fluid and slap it around in there. But On tanks that have thin walls or questionable spots where holes or patches are there, forget it thick POR is it. POR needs a higher temperature to cure, else it will sit there and run all over. Never even touch it unless its 80+. MEK will cure even in winter. Pretty well (my winter is the 60's so that is OK)
Kreem is designed for tanks in better shape than POR. That should weed out the choices for you. If you ever want to remove it. Kreem is it, and put it on very very very thin. Then you can remove it with MEK and stripper. POR they say cannot be removed and it should typically used where you need thick coating. So I'd expect one of those to never need to come off. Thin POR I am certain can be removed with a acetone, but I'll know for sure tommorow. I need to clean the tail gate of my truck.
BTW POR has lead in it. I'd avoid it if you didn't have to, too much exposure is terrible.