I have not tried LEDs myself, but I have seen them at a local fish store over nicely planted 20g tanks, and there is no question that the set-up in the store was doing well. I can't say if the units are the same. The one you linked has two rows of LEDs I believe, it may take two of these units.
I would not use the second link, the dual 24-inc fluorescent tube. That is way to much light over a 20g. If you decide on something other than the LED, the best is an incandescent (screw-in bulb) unit with two sockets. These hoods are not expensive, probably cheaper than the LED fixture ($39) but certainly no more. Then you get two Compact Fluorescent daylight bulbs from the hardware store. I use GE 6500K daylight bulbs over my 10g and 20g tanks, and I have two 10w bulbs over each and the plant growth is good.