Ikelite:
By far the least expensive, but if you build an extensive lens kit there is going to be some bulk for travel. Ikelite does not offer extension rings, so if you wanted to shoot 10.5, 12-24, and 18-70, you are carrying around 3 domes, and if you want to shoot a 60 & a 105, you are adding 2 flat ports into the mix... Glass fronts for macro ports, 6" diameter plexi- domes for wide angles, 8" port in the works for Nikon 12-24 & Canon 10-22. Ikelite has limited support for super macro setups.
Aquatica-
1 8" dome, plastic, placed by varying port extensions. Nice wide mount, so nearly any lens will fit. Macro port front is acrylic, which is a really poor material for a macro port (scratches easily, likely noticably more chromatic aberration). Good gear support for most lenses.
Nexus-
Many glass domes to choose from that engineered for specific lenses, as well as "generic" 170mm & 230mm acrylic domes. Athena, a third party manufacturer, also supports these housings w/ a 170mm glasss dome that is very highly regarded. There are basic flat ports (autofocus only), as well as flat ports with focus (af & mf with the 60 & 105 lenses), and the multiport which supports the 60, 105, 70-180, and 200mm Nikon Macros. The multiport is the ONLY setup that allows MF, Zoom, and Auto to Manual switching with the exotic Nikon 70-180 macro. My suggestion for people starting out with Nexus is the multiport, and an Athena glass dome for maximum versatility.
We also sell Subal, which has a very nice glass fisheye dome, excellent macro ports, and very well designed lens gears that are easy to mount and line up easily. Subal ports are actually milled from delrin to keep their weight down and retain near-neutral underwater handling for the housing.