Over the course of 20 years of fishing, I have gone through many batteries. Some only lasted a season or two up in WI. The last battery I bought was an Optima Deep Cycle/Cranking battery. Cost me quite a bit more than the cheaper batteries I had purchased previously, but it lasted quite a bit longer too. In fact, I sold my boat in 2008 to a friend of mine and when I went back up to WI this summer to visit, he was still using that same battery.
I'd say spend the money, you wont be sorry.