Someone mentioned plate thickness.
For 6V batteries, most you will find are the thick plate deep cycle type. It is important to buy them in pairs. They are only as strong as the weakest battery (an old battery will tend to draw off power from the stronger one as they sit pulling it down to the lower level)
12V batteries come in starting, combo or deep cycle. For trailer batteries, you want deep cycle if you go 12V batteries even if the amp-hr rating is similar. I don't trust the combo as they never seem to be clear about what trade offs they've made.