I don't actually have a 14 LED Duo, but I have an 8 LED and several 5 LEDs...
I'd be interested to know what people think of the 14 too. One was recently given to a friend of mine, as part of the Spelios helmet/light setup. We compared my 8 to his 14 in a cave, and the 14 is brighter - but not anywhere near twice as bright, as you might expect. It looks like it gives maybe 25-50% more light. I find the regular small LEDs without a decent reflector are only good for short to medium range anyway, compared to the 1 watt+ side-emitting LEDs that can be used for spotting. But the regular LEDs are still fine for walking passage, and the 8 is great for me. Maybe if you're in black lava tubes a lot, upgrading to the 14 might pay off.
I think the best thing about the 8 and 14 is being able to dim it. Stops you blinding yourself and others in crawling passages and when taking a break. The 5 LED can't be dimmed, but interestingly it's battery life far exceeds the others. It will last about 160 hours on 4 AAs, giving similar light to the medium setting on the 8 and 14. The LED inserts, by the way, are really easy to install - and they fit all vintages of Duo as far as I'm aware.
My conclusion - I'll stick with my 8 for now. I got the insert for about $30 a few years ago, when a store had one left on clearance.
FYI - I use the 8 on my regular helmet setup, with a Princeton Tec EOS 1 watt LED that runs on 3 AAAs as my spotting/backup light. My carbide setup, which gets little use, is an Aceto ceiling burner with a Duo 5 LED as the backup. 8)