I have the 6' dia plastic pond right now it's in my pics. I think you would be good for some time with it. My biggest fish is pushing 29" and has more than enough room. They are 350 gal. 2ft tall and 6ft dia. I would go for it if you can. Once the fish in it gets to around 3 1/2 - 4 ft. it will be time to upgrade. Mine swim laps around it for hours I put the water flow going around the outside and they follow it like a stream. I would say go for it. I am not too sure about the galvanized tubs, but I am not willing to risk any metal leaching on my fish. as for pims that get that size you are looking at they are the most expensive ones. I would say a rtc would do good for probably 4 or 5 years in a 6 ft. dia tub, really just depends on the individual growth rate of the fish. They can vary alot, but that tub would buy you a lot of time. With the 3ft. you mentioned 2 things, you are not going to have nearly the amount of time as with the 6 ft. dia to grow out, and you have a possible metal hazard, and you don't want your fish smashing into metal that doesn't give much. That's one thing I have come to love about my plastic tub is that the fish run into it as much as they want and no major damage. Glass and metal when fish run into those they can break bones and damage fins whiskers ect. Most I have had with the tub so far is bruised noses, but I am sure that's from the huge stones I have for substrate. Sounds like you are anxious. If you are really willing to keep a rtc for life and can afford the food bill and tank maintanence go for it. Get the 6 ft dia tub, caution they are tight fitting down narrow basement staircases. I had to squish mine a bit to get it into the basement, but with a lil effort it worked! Just remember eventually you will have to upgrade. I am moving soon and hope to get a place with a garage that way I am virtually unlimited in space for the pond. I am going to get the 8 ft dia tub if I do get a place with a garage! They are still 2ft tall but hold 500 or 600 gals.