And I think one 35w bulb would be adequate. You run into issues with
shallower tanks and T-5 lighting, but for your dimensions one would be
fine.

The tank is about 18in deep.

It all boils down to your setup- are you planning on running CO2? If so a
two bulb system could be best.

The current tank has a DIY Co2 system.

If not, go with the one. I run a lovely planted Discus tank with 1.2 wpg of
power compacts- this is referred to as a
low light/low tech setup and maintenance is a breeze.