Is there a significant difference in the final product between these two?
You should watch episode #7 of the Meadology series, by Canadian Sasquatch, on YouTube. He explains the basics of SNA (staggered nutrient additions) and YAN (Yeast Assimilable Nitrogen).
Basically yeast need a certain amount of YAN during fermentation. There are equations for calculating how much YAN you need for a specific yeast and batch size, TOSNA and TiOSNA being two of the more recent.
Roughly, DAP has about 200 ppm of YAN, Fermaid K 100 ppm and Fermaid O 50 ppm.
The trend seems to being going towards Organic nutrients (Feramid O), which is a little more expensive, and away from DAP and Fermaid K (which also has some DAP in it). But what helps offset that cost is that, when using the TOSNA protocol, you end up using approx. 1/4 the amount of Fermaid O. This can get halved again if you end up fermenting with fruit.
So, you also need to check out the meadmaderight website.
All that being said, some of the best mead makers in the US are still using DAP and nutrients with DAP in them. I "think" one of those is Ken Schramm. Personally, when I started , I bought a big bag of DAP and some Fermaid K, and so will continue to keep using it until it is gone. It's probably more important to be working on my technique than worrying about whether I can taste the difference between using K or O.
Oh, and from my limited expense, Fermaid O seems to ferment slower than Fermaid K and DAP.