When it comes to buying a tank, bigger is always better. While I would love to have a 500 gallon tank I do question the cost of upkeep (salt, additives, food etc etc). Personally between 180 and 300 is my sweet spot. Most of the gear can be found locally and it isn't a killer to maintain.
Honestly stick with the 20, the equipment to get that running is a lot cheaper than a sumped system. If you stick with it you will want to upgrade regardless and which point you can sell it and buy something else thats more along the lines of what you need. Id say after about 1.5-2 years you will know if you want to continue and go bigger or quit.