The primary reason I can think of for having the larger battery is if you spend a lot of time listening to the radio with the engine off; otherwise, the PC545 is just fine. Either way, it's nice to have a sealed battery with no corrosive seepage. No metal jacket required.
If you leave the car unattended for any significant time (e.g., > a month), the alarm system will drain down the 12v battery. I now plug in a small (very smart) Battery Tender when I know I won't use the Insight for even a couple of weeks.
Because our Insight's 12v charging system via the dc-dc converter is surprisingly unsophisticated

, you might consider using a modern three-stage charger to occasionally rejuvenate the 12v battery (be it the PC545/PC680 AGM (be very careful not to exceed the upper voltage limit) or the OEM flooded-lead-acid).
Do a Forum search on "PC545" and you'll find some in-depth discussions and installation photos. HTH. JoeS.