Will you be using the same battery for both your starter and aux? I am leaning away from a typical isolator to charge my aux under the hood. I am rather interested in a DC-DC charger. This will allow me to use different types of batteries (lead acid, agm, calcium, starter, deep cycle, etc...) I really like Redarc's stuff out of Australia.
Before I do the dual battery under the hood however, I'm currently building my "camping" battery set. For starters, I'm mounting a class/group 31 in the rear cargo area above the drivers-side wheel well. For odyssey, this would be the PC2150 and for the Sears Diehard Marine, this would be the 31M:
http://www.sears.com/diehard-platinum-marine-battery-group-size-31m-price/p-02850131000P
I'm going with the Sears because Odyssey builds this battery for Sears to the same specs as their PC2150 for half the price. Plus, I like the blue color as some of the battery will be exposed in the mount. This battery is going to be for my camping needs but primarily, just for the ARB fridge. With my math, If the fridge draws on average 1.2 amps, I could run it off this battery alone for 41 hours before the battery reaches 50% capacity with zero charge input from the vehicle alternator or solar panels. Eventually, I'll add a second matching battery on the passenger side to double my power.
Back to your topic thougn, I plan on putting (2) batteries under the hood. The auxilliary under the hood will be for all vehicle accessories (lights, radios, etc...). If I don't end up using the same type of batteries under the hood, I'll get a DC-DC battery charger. If I find (2) deep cycle batteries that can put out enough CCA to start my Jeep, then I'll just get an "auto" isolator like the one in the Genesis kit. The disadvantage to a DC-DC charger is that it offers no "manual connect" to jumpstart your starter battery. This is simply solved with a 200 or 500 amp isolator however.
For my camping battery, I'll be using Redarc's BMS system. It's a really cool unit that will give me a small LCD screen to mount up front giving me live diagnostics of my "camping" battery bank. I won't go into detail about it, but it's really cool. It even offers an AC input, though it only accepts 240V which we don't have here in the states. I would have to get an up-converter to be able to plug it into a wall port at my house. Not a big deal as I don't plug my vehicle in a lot.