i like real plants MUCH better. they look very nice and if you get java fern, they grow very easily, no plant food or anything required. just keep the flourescent bulb on for at least 8 hours per day and it'll grow very nicely.
also, plants help fishless cycling very much plus they grow algea and you could get 2 ottos for the tank. they'll be active, swim around, eat the algea off the plants and glass and the betta has no problem with them.
as for substrate, i have blue gravel and its great, i added 3 beach rocks i found too for decoration.
marbles i think are too hard to clean, the suction from a syphon won't be able to lift them and dirt will still be stuck. thats why i think smaller gravel is better, but its your call.
also, don't get undergravel filter, they are bad from what i hear. the flow, hanging type is much better. i have a small one for the 5 gallon and water is always crystal clear.