Hi Paul,
Like Damien said the whole Nikon vs. Canon thing is really a personal preference. I've shot both and both do the job well. So I won't say much on that. Currently I have a D300 in an Ikelite housing with 2 DS125.
But on the stobes, since you already have a DS125, I think adding a 2nd would be a nice way to go. It is true the Inons are small and light, great for packing. But I like the warmer light from the DS125, if I remember correctly it is 4800K vs. 5500k for the Inons. Your pictures will have a nicely blue water background shooting WA. Also I love the 1 sec recycle of the DS125, it is just fast. The newer DS160 is 1.5 sec I think. The Inons are powered by 4 AAs, while the Ike battery packs are 6 Cs for a lot more juice.
As for housing, if you want TTL, and already have the DS125, I think it will save you a lot of money to go with the Ikelite housing with the TTL built in. That's basically the chain of logic I followed when I brought my Ike housing for the D300 instead of going with Aquatica or Nexus. I like shooting TTL for macros and I don't want a big, expensive external TTL converter. One more point of failure and more drag in the water.
My 2 pennies worth. Hope that helped.
Cheers,
alan