[MENTION=26538]steel_is_real[/MENTION] - The Trek 730 has a bit nicer steel tubing than the 720 and the 730 came with a bit nicer components, in general. They have 700c wheels vs the mountain bike which you said has 26" (smaller) wheels. You'll want tires for street riding not heavily lugged mountain bike type tires on asphalt/concrete.

The Trek 7xxx are aluminum and not steel.

Will you do your own work? The most common things to upgrade/change are the consumables, i.e. brake pads, cables & housing/ shifter cables & housings and maybe tires (if they're old I'd personally replace because rubber degrades). Size is the most important thing. How tall are you? Do the sellers say what size the bikes are?