I do not have a Vortech so I can't comment on them. I have a 29 gallon biocube and just switched from one Koralia 2 to 2 Nanostreams so I can comment on a difference or two that I notice between the two.
I overall liked the Koralia 2 as it was a good pump and I never had any problems with it. I never liked the overall size of it since it was pretty freaking big in a nano :). I decided to go with the 2 Nanostreams because of their smaller size, increased flow, and over performance and reputability....also they were 15% off at the time so I got a good deal on new ones.
The first thing that I like is their compact size compared to the koralia. They are probably comparable to the size of the Koralia Nano or K1 so it made my tank feel that much larger.
Second thing that I noticed is not only the amount of flow, but the overall difference in the spread of the flow. It seemed like to me that the Koralia was nice and diffuse but it was still pretty linear in one direction. One thing that I noticed right away with the Nanostreams was not only how diffuse yet turbulent it is, but also the overall randomness that I am able to create with them.
I am really happy that I made the switch and so are my SPS :)