New and need help

[Ruth]

I have to agree with BMW go as large as you can afford. The smaller the tank the more prone it is to imbalances in the system. Even something like putting your hand in there to replace a fallen rock can have very negative effects if your hand was not clean.
With that said it looks like you have narrowed your options to 2 so I would suggest to go with the 20g and just take everything really slow. Talk to other reefers either in person or on this or other boards - do lots of reading - there are lots of links on here and if you just ask Beverly will give you plenty. If you get a chance go and see other peoples systems and see what works for them. And my #1 recommendation is do lots of water changes - especially on a small system. It's easy cause your not dealing with a lot of water so weekly 25% or more water changes are not really a hassle.