I had a similar decision, house my 1DMkIV or get something dedicated for UW. I went with an Olympus EM-5MkII. The price for the housings was a significant difference, in Australia a Nauticam housing was $7k and the EM-5 MII $2.2K. The DSLR housings are much bigger and heavier and harder to push through the water. DSLR lenses also require big domes for Wide angle. More weight, carry on size, effort in water.
Speaking of water it's a bit of an equaliser, needing to place your wide angle glass behind a dome port robs it off sharpness, there are threads around the new Nauticam WWL and WACP lenses, the whole premise of which is that the dome is the limiting factor, the claim with the WWL is that a 14-42 kit lens with WWL out performs the likes of a 7-14 pro lens in a a big dome. Similarly the WACP is built around small kit 28-70 f3.5-5.6 lenses. The claim is that the degradation caused by the dome is enough to bring the pro lens down to the level of a kit lens, but placing the kit lens behind the WWL it performs as well as it does on land. I'm more than happy with the oly lenses and I have L glass for my Canon.
I can carry on my OM-D Nauticam housing with tray/handles, 170mm Zen dome, OM-D camera, 12-40 f2.8 zoom, two Z-240's plus my 1DMKIV and 300mm f4 in a regulation size domestic carry on, fitted out with padded dividers. That's what I took to Ningaloo earlier this year. When I was buying my system, the person in the shop told me they didn't even try to carry on their DSLR rig, but were resigned to placing it in a Pelican case and checking it. With my setup my flash arms, clamps, chargers etc get checked with my dive gear.
I've now got an EM-1 MkII (started with EM-5 MkII 2 1/2 yrs ago) and it runs rings around my old iDMKIV in some areas.
What type of shooting are you proposing to do? wide angle? macro? both? tropical or temperate mostly? I've found for temperate shooting round Sydney year round the 12-40 lens does great, I also have the 60mm macro for nudis etc.
I'm selling my EM-5 MKII Nauticam housing and camera if that's of any interest. housing, camera, tray, handles for the price of the bare housing new.