I've been using play sand for a long time now, and I have had no issues planting in it. All you have to do is add some root tabs and plants grow like weeds. All 3 of my planted tanks are sand only, and unless I have a particular 'scape in mind, I have no reason to switch for now.
A $3 bag of play sand will be way more than you need for a 20 gallon. If you're just trying to learn and figure out if the hobby is going to work for you, then maybe it's a good idea to try sand before dishing out $100 for expensive substrate (shipping is expensive, too).
The only issue with sand is that if you have too deep a layer and you don't 'mix' it every once in awhile (every 3-4 weeks or so), anaerobic pockets may form (anaerobic bacteria proliferate and produce toxic gas). Those are bad, because once the bubbles rise to the surface, the toxic gas can kill your fish. That being said, I only have 2" of substrate and I've never seen any anaerobic pockets. The deeper the sand, the more serious the issue though.
The type of sand you get really doesn't matter though. I guess it depends on what look you prefer. Just make sure to rinse until the water runs clear :-)
Edit: if you don't want to stir the substrate yourself, just get some malaysian trumpet snails. They'll do all the hard work for you :-)