If resolution is important to you, I would not go for APS-C (although ok as a temporary solution while waiting for 5Dmk3 or something). Pixel sizes on top end APS-C (16 - 18 megapixels) is so high that they outresolve most lenses with a bit. This is no disaster of course (it is kind of good that the sensor does not set the resolution limit). However in practical prints you'll find that about 12 megapixel fullframe makes as sharp prints as the 16-18 megapixel APS-C, when using the sharpest lenses. When using less sharp lenses 12 megapixel fullframe will even be sharper in comparison.
Concerning prints, good printers/software are nowadays very good at hiding pixels, that is you can print (almost) as large as you want and you won't see any significant pixel artifacts (that is you don't see individual pixels when looking close), the print just gets a bit less sharp. How unsharp prints you can accept is a matter of taste. APS-C on 20x30 will look "good", but 21 megapixel fullframe with lens will be sharper. Personally, I would think APS-C is too unsharp for that size. 20x30 will only get ~160 ppi, and for APS-C those pixels are not very sharp either. With APS-C I would recommend not to print larger than A4 for book prints (really close and critical viewing distance) and A3 for framed prints (a bit less critical). Larger than that you will compromise quality, but if printed right it will not look "ugly" just fuzzy.
People have very different opinions on resolution needs of prints though. I'm one of those that prefer rather high resolution, and would actually want medium format 40 megapixels or more for prints as large as 20x30.