Keep both, but make each distinctly different to keep the interest levels up there - I actually have 7 marine tanks and 8 freshwater tanks, the biggest being a 55 gallon planted. The marine tanks are all nanos: 2-20s, a 15g, a 10g, a 5g, a 3g, and a 2.5g - they are all different (biotopes, softies, LPS, etc.) and that is why they are still running!
Actually, the 10g and the 5g are essentially the same other than size (large tree corals and zooanthids), but one is at home and the other at the office...
I do admire the people that simply run one well-done system, it is just too bad I am not one of them!