I think the most important factor is that the Blue Seas add-a-battery unit, the batteries are two separate circuits. 1 bank is dedicated for starting. 1 house bank for everything else. Both charge while running but discharge independent of each other. The only time they are combined is on "both". Both would theoretically be used only in case of emergency starts.
If your batteries are always combined, and you have a bad cell or whatever, the bad battery will drain the good one. If that happens you can switch to 1, 2 or both and your still SOL. With the blue seas, a diode keeps the bad battery from draining the good one.
I'm sure you've already read this:
http://bluesea.com/viewresource/92
I would definitely call Blue Seas 800 number and talk to their tech dept. They will explain the pros and cons much better than I with out trying to sell you on anything. From what I've seen, their design and engineering continue to evolve while perko's been making the same mediocre stuff for the last 30 years or more.
The last two Perko battery switches I've had, failed. When I first saw the Blue Seas product my first thought was "how do the other guys dominate the market, when this is so clearly superior in overall design and quality?
Good Luck