Get the tank, then the "tank buster", things happen in life that scupper the best laid plans (of getting/making the big tank) and it is the fish that will suffer.
I have a Giraffe Catfish (hopefully Auchenoglanis tchadensis but it could easily be occidentallis), bought at ~5cm SL September 2012, now ~15cm SL and so is half adult size at best, growing up in a ~280l 4-footer.
I have an Ornate Bagrid Catfish (Chrisichthys ornatus) bought at ~4cm SL September 2012, now ~8cm SL so is a third adult size, growing up in a 140l 4-footer.
I have 5 "African Suckermouths" from the Euchilichthys group bought at ~10cm SL last September, now ~11cm SL and will reach ~20cm SL eventually, growing up in a ~300l 6-footer.
I have a Synodontis notata bought at ~22cm SL February 2012 which is now perhaps ~23cm SL, with only ~2cm of extra body length to go, again in the ~300l 6-footer.
But I have the 6-foot tank and an unoccupied 5x2x2 in the garage amongst my 8 tanks.