Why not get the 20g get a fiji cube AIO insert and lessen the need for a upgrade ? Get a simple 10g from wal-mart for a QT tank with a aquaclear filter,heater and maybe a small power head (sicce .5 $25) for extra flow ?
I just don't see why people spend the money on small tanks getting into the hobby then eventually upgrade. Go bigger,less room for quick changing water parameters,getting too big of fish that need more water volume than the small tiny tank they put them into.
Make it easy on yourself. 20g is a good volume to start at I feel like for new people. I started with a 10g then went smaller once I learned how to control parameters. Now all I keep is pico's,because they are a challenge.
I have one been in the hobby for 12yrs so take my advice with a grain of salt !!!