To be completely honest, I would not take on a betta if I had to keep it in a 1g... I have thought about it-- I still have Lou's 1g bowl but that would just be irresponsible to knowingly neglect an animal. Don't get me wrong, a 1g can be feasible but only if you can change it every 2-3 days, heat it and provide enough mental stimulation for the betta. But unless you are a Taiwanese betta breeder, this is probably more trouble than it's worth!
Be aware that 1g should only be attempted by experienced fish keepers IMO. Like DQ mentioned you really
have to keep up with the water changes, temperature and generally know what oyu are doing because parameters eg temperature, pH and ammonia change really fast in a 1g bowl. You will do yourself a huge favor long-term by just going with a 5g from the start.