IMO Kessil look a lot nicer, but reefbreeder may grow better.
I have 3 Kessils over my 180 mixed reef, they seem to grow everything fine, from softies, to sps however I also have a 40 gallon anemone tank plumbed into it with a reefbreeder value fixture over it. There is just a carpet anemone and some softy frags but the GSP fragged from the 180 has actually overtaken it size wise. Zoos and palys seem to grow great as well. The carpet anemone was bleached but has regained its colour. I am really surprised these fixtures are as nice as they are. The Kessil just looks better though.
When I upgrade my tank to an 8 footer I will likely go with 4 Kessils and 3-4 reefbreeders.