The 28watt'er uses fractional leds (less then 1 watt each) while the other uses 3w
led's.
The fractional leds just have no penetration and only the top leafs will get light.
*would work with small herbs and maybe lettuces?
Most people (from what I've read) suggest 3watt LEDs b/c the better penetration. I personally use 1w led's (b/c they were cheap/easy to build) they seem to do fine on my small plants (less then 2 foot tall).