If you decide to get a 5 gallon tank and cycle it you can split it (like I'm planning to) And get less work than your current 2.5 gallon tank! How? Becuase than you can keep both Bettas in it and still have each get 2.5 gallons, and if it's cycled you never have to do 100% water changes, so no washing out the gravel, plants, ornaments, etc.!
You could just have your current 2.5 gallon tank as back up in case something occurs. Heres the link to a 5 gal that I like that you may like:
You'd probably have to do work until the tank is cycled. The method of cycling I'm using that's simple to do is:
1. place raw shrimp in pantyhose/mesh bag (Nothing with dye!) and place in tank
2. Once ammonia reaches 8 ppm remove shrimp
3. Once ammonia and nitrites are 0 ppm and nitrates are 10-20 ppm do a 50% water change (NOTE: don't change the water at ALL until this step)
Simple, quick way to cycle tank, simple tank with less care than your current one. You'd probably have to make your own dividers, though, because most don't seem to work well. As I'm almost certain you know, no 100% water changes in a cycled change EVER unless some emergency occurs, because that ruins the cycle.
Hope I helped!