I think much depends on your anticipated use.
If you want something that will work with minimal to no intervention, and you'll be wiring accessories to the secondary battery directly, you can use a solid-state isolator (i.e.: Hellroaring - I've gotten the most consistently positive reports from the field on their units,) wire it up, and forget about it.
If you want a flexible system where you have to keep track of what's going on, go with a constant-duty solenoid (what rating depends on a number of factors.) However, a CD solenoid allows you to wire up batteries en banc and use a trigger lead from the vehicle to automate switching, you can wire in an "override OFF" switch (to cut out the secondary battery entirely, in case it goes flat and you need to start without interference - or it goes dead, and you need to just cut the thing out until you replace it,) an "override ON" switch (so you can self-jump, with a good accessory battery,) and you probably have more flexibility in terms of how you can add, configure, and use more than one additional battery.
Thumb rules for multiple batteries:
- Batteries wired directly en banc (constantly connected in parallel) must be of the same type, capacity, and age (as evidenced by the manufacture date code tag on the battery case.)
- Batteries of dissimilar types, capacities, or ages must be "isolated" from each other when not under charge, or you get "duelling battery" syndrome (the stronger battery tries to charge the weaker, the weaker doesn't accept the charge, and both batteries end up flat - with the possibility of being wrecked.)
- Banks of batteries may have different numbers (for instance, one starting battery and three deep cycle auxiliaries,) but the provisions in the previous two points must be observed (the three deep cycles must be the same type/capacity/age, or be isolated from each other. This may be a case of "mixed isolation" - the starting battery is isolated from the deep cycle bank by a solid-state isolator, the deep cycle batteries may be manually isolated from the rest of the bank by switched solenoids. This allows you to cut out a "dead" deep cycle and preserve the capacity of the other two - but the three should still be replaced all at once.)
A "solid-state" isolator is essentially a high-current diode pack/network. A diode is, simply put, a "one-way valve" for electricity, similar to a "check valve" as used in fluid power. Like a check valve, a diode will have limitations on reverse voltage (pressure) and current in either direction (flow,) and the higher those two specifications are, the more they cost.
A constant-duty solenoid is essentially a regular manual valve - it allows full flow in both directions, when the terminals are closed.
N.B.: A "Ford starter solenoid" is not a constant-duty solenoid! I know RVers have been using them for years as "isolators" - they're cheap and readily available. But, they never seem to understand why they have to replace them every six months, when the coils burn out (no matter how patiently I tried to explain it to them, back when I was holding down a parts counter...)
"recon" (sic - reckon)(tm) "hihgly"(tm) "seceed"(tm)
"Outback AIDS - Alcohol-Induced Dizzy Spells"