Its your personal preference really. if your betta is a real showstopper, a very plain and drab look will make it stand out. If you choose glitzy marble type glass substrate, then matching decoration might compliment it.
I personally am a big natural look lover. Terra cotta pots are $.75 each at wal-mart. And they work great. You can smash them up into little pieces and stack them, leave them all whole, mix and match. Whatever you do with terra cotta plants, either cover the holes in the bottom, or break them open wider. Your bettas *will* get stuck, otherwise.
Live plants are my fancy.