Freshwater. If price is an issue, then even just maintaining a SW
tank will cause some agony, especially with getting a 15g up and running, it's tough to get a stable small system running, especially without a large refugium attached.
There are still some very beautiful FW
tanks out there, especially the planted tanks. You can also consider an african cichlid tank too.
The other benefit of the larger tank is if you'd like to eventually like to convert to a SW
down the road, you already have the tank and some of the equipment.