I've got a 55# Terrova w/ IP on my PT175, and like it a lot. If you fish brush piles a lot, then the spot lock will not hold you in one spot like 2 anchors would. You will drift off and then it will pull you back to the spot, not the exact same spot and maybe/maybe not in the same position relative to the brush pile. I use the spot lock to keep me off the bank or whatever when I want to get a drink, re-tie my rods, change lures, etc;. I don't worry about anything then, but you do have to make sure you don't accidently turn the spot lock off and start drifting, etc.

55# will be plenty for your boat, you could run 2 batteries parallel if you have room in the boat for more time on the water.