12V and 14V -or- 24V and 28V are used interchangeably to refer to the same systems, so there are really only two options. Traditional lead-acid batteries have a nominal voltage of 12V, but vehicle charging systems operate at nominal 14V (in both cases, those numbers are rounded by a few tenths). Likewise, the other option is a 24V battery in a 28V charging system.

The advantage of higher voltage systems is that the builder can use lighter gauge wire to deliver the same energy to a device. For example, a 50W halogen landing light will draw ~4.2A at 12V, but only 2.1A at 24V. In a large aircraft the savings in copper wire weight can be significant, but in something the size of a Kitfox it won't even offset the added weight of a 24V battery.

Battery choice is the other issue. There are lots of options for small, lightweight 12V-ish batteries, but I'm not aware of anything suitable for a Kitfox at 24V. You could put two EarthX batteries in series, but they might not charge properly in that configuration; I would ask EarthX before doing it that way.

Finally, take a look at the electrical section of the 915iS installation manual before you choose a battery. Depending on the mission you're building the plane for, you may want more capacity than the ETX680C offers. See my longer post on this subject, here (there are several other useful posts in that thread as well).