Or wave. I think the distributor is called something like J and k or something for both. Most pet shops used to carry them but I've not seen anything by superfish in a while!
I would think 18w is less than 2x11w in terms of light, it should be fine, failing that dennerle are proven quality but not cheap, easier to get your hands on though. Thinkg about it, Tom Messenger if he's still active was using a aquael light I think?
Fluval have an 11w on the market but the only one I've seen I didn't think the quality was very good, the light was noticeably flickery and didn't seem as bright as dennerles.