First of all Matt, check your outboard motors owners manual as some motors (like my Optimax) have required minimum battery specs for your house/starting battery. For this battery I'd suggest either a group size 29 or 31 but certainly not smaller than a group 27. Depending on what group size is currently installed in your boat, you may or may not need to change out the battery tray(s).
As for your TM, make sure it's a marine deep cycle & designed for TM use.
Brands are like the Ford vs Chevy debate. Go with the best battery your budget allows. Some say the biggest bang for the buck are the Walmart EverStart Maxx group 29 size. Interstate Batteries have been working good for me & is what came in my boat in a group size 27. I have read that the Sears DieHard Platinum marine batteries are good but are $$. I put an Autozone Duralast marine battery in my little tin boat & has been working good.
So my thoughts are, get the proper battery for the application in a larger group size with finding the brand that works for you.....
Oh, and whether it's Sears, Batteries Plus, Costco or your local Interstate dealer I'd buy close to the house as you'll be visiting the store again in +/- 3 years.....
LLL