Hi Terry, Where are your batteries situated? I have 2 group 27 AGM in a hatch port-side of motor of the 21.
I think the AGMs are the better choice They can sit with out losing a charge for a good long while (ask me how I know)
You might check with a local battery source. Call a golf course and find the source of their golf cart batteries -they'll carry a variety of rec vehicle stuff.
I bought some Deka/East Penn group 27 for the Seafari for $158. I think the 24s were $125 or so. Been real pleased. The agms Cabelas was selling were Chinese (Universal power group). They are fine, too and Cabela's customer servive will always make things right if they product is problematic.
The I/O will recharge better than my 9 amp Merc 115 so the 24s will probably be fine for the Stroker but you know you'll want the extra CCA of a starting battery to turn over the Stroker.
I am using the Blue Sea Systems add-a-battery switch with the acr/battery isolator which works great. I have some time this week so I'm back on the 21 project...