Dan,
You save weight either way. You need a field approval to put an Odyssey in the original place..... I got mine through Cubs Unlimited, and he also did a great job of fitting the battery and making a foam filler insert to take up the now extra space in the battery box.
I used it this way for 2 years, and the battery worked great -- no starting problems ever. This little battery really cranks the engine over! And some people like the battery in the original place, it puts weight just where you need it if you don't already have a lot of weight in the back.
However, now that my cub is undergoing total rebuild, I decided to clean up the configuration and get rid of some cable. There are two under-seat battery STCs, Dan's Aircraft and Atlee Dodge.
Atlee Dodge - you get to keep the springs in your front seat, probably more comfortable for this reason. However, there is no place for the solenoid, you need to find a place to mount it yourself.
Dan's - There is a nice place for the solenoid, everything is mounted in one compact package. However, you need to take the springs in your front seat out and this gets replaced with a metal lid that fits over the battery compartment. I was told that this is because there is a possibility in turbulent air of the seat springs hitting the battery terminals and shorting it out - apparently it happened once or twice before.
I selected Dan's kit just for the more compact package with the place for the solenoid, and am crossing my fingers that I won't lose too much seat comfort on the long cross country trips.