My vote goes for heavily planted, with a higher number of smaller (but not necessarily small) fish. Part of this depends on your budget, of course.
I think a 125 would look incredible with an array of various Australian Rainbows. There are so many colors to choose from, so you really get to pick what you like best with those guys. They're also very active and normally not terribly timid, so you'll really get a lot of bang for the buck out of them. With a tank that size, I'd also do some research into some of the fancier pleco species and pick one (maybe two) of the really nice interesting striped or spotted species. I'd fill in the rest of the tank with at least one or two species of killifish (taking care not to mix species that might interbreed). For instance, a nice fundulopanchax species for the mid to lower region, and then an epiplatys for the top region. These guys tend to breed like crazy, but the rainbows would help cull the numbers. Finally, the real clean-up crew. I'd pick a nice schooling species of smallish loach or if you're more inclined towards cories, a school of those. Oh, and the icing on the cake -- a plethora of interesting invertebrates. Rabbit snails, maybe mystery snails, some nerites (they really help with algae), and shrimp like cherry shrimp and bamboo shrimp.
Hopefully that stocking level would leave me room to fill in with new acquisitions now and then, so I could pick up something new that happens to catch my attention or I could try something new to replace the fish that pass away over time.
Seriously though, what's your ideas?
