You have to consider Terry's questions and have answers before you go buying, because you will expect the battery to last a little while!
The general rule of thumb with batteries is to drain them to no less than 50% of their capacity. Below this point the batteries start to sulfate rather badly and their life expectancy (in both meanings) is drastically reduced.
So - work out how many amps you're going to use for what time you expect to use it. Be generous in your usage, because having a little extra capacity is always better than not.
When you've worked that out, double it and buy a battery that large.
If you have a decent charger and you don't mistreat the battery it will last you for a few years (let's say 5 years). Take the cost of the battery, divide by 5 and see if that number is more appealing to you.
Our 100Ah $300 gel (it's a little more, but I don't remember exactly) isn't bad at $60 a year.
Next time though, I might consider a larger capacity AGM. 100Ah is really only just enough to bring us through a night.