For that low of a budget, I would stick with what you have and wait until you can afford an upgrade. Getting basically the same level of bike with the only difference being the size of wheels wouldn't do it for me. I I bought an old '97 Gary fisher Wahoo rigid and fixed it up and will ride it until I can spend enough to get a great bike. Are you having fun on the bike you have? If yes, keep it until you can really upgrade.