They are definitely under sexual selection. As yet we have no idea if they involved in species recognition. I think not. I think it can easily be both at the same time. I think it most likely that sexual selection leads to the variability between species because it is a process which is likely to arbitrary in terms of which traits are selected for.
Theory suggests it can often spin off in unpredictable directions.
Selection for species recognition per se is likely to be strongly stabilising and thus cannot easily produce variability between species. And its entirely discussed in terms of colour, although the logic could apply to any male trait at all. This would be a testable hypothesis. You would predict that males within a population of a particular species would have more variation in colour in bower builders than in non-bower builders such as mbuna. This is simply because all males of bower building species are more brightly coloured than the females.