The Fenix is a tightly collimated spotlight, which means it will have a high apparent brightness but it only illuminates a small area - it is sufficiently water resistant for almost all caving that won't involve submersion. The Petzl Duo is antiquated and therefore very dim, but easily upgraded with modules made by [url]customduo.co.uk[/url] and quite waterproof if you take care of it. If you can find any generation/model of used Duo on eBay in good shape for around $30 or so and upgrade it with a CustomDuo module you'll have a very affordable, versatile, and bright caving lamp - but I wouldn't suggest anyone spend more than that on a Duo these days. I doubt that the Fenix will seem like much of an improvement over the Apex you have been using, and unless you upgrade the Duo right away it will seem like a big step down.
If you haven't seen it already, check out the Caving Headlamps List spreadsheet I posted here:
http://www.forums.caves.org/viewtopic.php?f=2&t=14881. You might want to consider some of the only-slightly-more-expensive but not-made-of-plastic offerings by Spark and Zebralight that use the newfangled very bright and very efficient XM-L diodes being made by a company in North Carolina named CREE INC.. Spark's SX5 model is a very nice choice and uses AAs or AAAs or 26650 size lithium-ion cells, and Zebralight's H600fw model is awesome but only uses 18650 size lithium-ion cells. Lithium batteries can have quadruple the energy of twentieth-century style batteries but do require slightly more care regarding safety, mainly when charging.