I haven't used the micro-jets, but have used the mini-jets. If you look at the dimensions, the minis really aren't that much bigger than the micros.
The one thing I really liked about the mini-jets is that the flow rate is fully adjustable, where as the micro-jets only have 3 set flow speeds. With mini-jets you could adjust the flows to suit your needs.
I think 2, or even 3, mini-jets would not be that noticable in a 20g long.