If your sole use is to solder tanks, probably the bigger the better, but to be honest, soldering is a function of technique more than it is equipment, and I have seen incredible jobs using anything from an American Beauty 600W uncontrolled industrial from the 40's to a 15W pencil iron from a convenience store tool display. The single best tank workmanship I have ever seen (and what I aspire to) was from Ed Robbert and he used a Weller or Unger 15 or 25W pencil iron normally used for PWB electronics work. It looked like it was cast from a single ingot of tin, the seams were nearly invisible.
I have 6 different soldering devices, a 350W American Beauty industrial iron from the 50's, the big Weller gun, a Weller temperature-controlled station, an ancient Ungar station with temperature control in the tip (and am still using the tips that my dad got in 1966, still all in good shape), a really inexpensive Weller 15W pencil iron that I got when I couldn't afford anything else, and one of the butane mini-torches that I used to carry around like Floyd, but haven't seen for about 20 years. I use the Weller gun exclusively for tanks, I use the Weller station for essentially all my electronics work (very extensive, for the last 25 years), very occasionally use the American Beauty for soldering something to radio or audio equipment chassis, never use the Ungar (it's a backup), little Weller (although I used it for 15 years for everything), or the butane torch.
That's different from brazing, I use a MAPP gas torch for that that I got at the hardware store, about 5 minutes every 10 years.
Brett