I'm getting into restoring old honda CB's which always have bad tanks. I have used both Kreem kits and POR-15. I would recommend POR-15, it does not make a coating over the metal, it actually binds and hold on to the metal. POR-15 has been around forever, and most car restoration folks use it for rust removal and prevention.
Kind of funny too, POR-15 sells a "Kreem Removal Kit" which basically removes a failed Kreem coating and applies their stuff. If that speaks for itself.
From my experience, I follow the direcitons to a T, the Kreem works well if the tank isn't real rusty, but if its really bad, the POR-15 holds much better.
-j