Sam, in recent years, I only use/used lead/acid size 27 deep cycle batteries and the brand depends on which has the best price. I've used Deka, Diehards, Delco, Everstarts, Stowaways, Interstate and several others and most last about the same.
I've used the group 31 batteries before for single 12V use, but the weight and/or size became prohibitive.
Yes, you can parallel batteries to get more capacity, but your best usage of two 12V batteries for a 12V motor is just using one until it runs down too much and then use the other. For electric waters, it may save you from paddling back to the ramp some day if you use the backup method. You can venture as far as one battery will take you and have some assurance that you will have enough battery power in reserve to get you back. That is particularly advantageous on a cold, windy day.
Most of the waters I fish are electric only so battery capacity is important to me and that carries over to bodies of water where gas outboards are allowed. Even when I fish gas outboard waters, I tend to fish a selected area all day with the trolling motor rather than run around with the gas motor. If I decide to fish for 15+ hours (I do that a lot in the summer), I know I have the battery power to do so.
My present aluminum bass boat has 4 Everstart batteries and one Interstate battery (came with the boat) and all are size 27 because that's all I had room for.
If you have ever tried to fish Diascund Reservoir for a long day, you should know how important having some power left at the end of the day is. That is no small pond.