Whatever you decide, install a Battery Mat first to soak up any leaking acid. The Battery Mat is a treated fiber mat that retains and neutralizes acid. But keep an eye on the terminals for that white corrosive powder because it's a warning that acid is likely permeating the battery cable and heading toward the starter.
A gel-cell battery is the best alternative. Optima and Exide have gel-cell replacement batteries that fit directly into a C5. The only concern could be the top terminals that come with the dual-terminal batteries. If your C5 has a dual-terminal battery, keep the covers on the top post terminals to avoid shocking or welding.
The benefits of a gel-cell battery outweigh its cost ($110 to $130), and the warranty is slightly better. Most have 84-month prorated coverage, with the first three years at no cost to the customer for replacement. Use the Web sites listed below for comparison and facts about the gel-cells.