I'm assuming your current ones are flooded or FLA. In that case 6V GCs are generally superior to most 12V varieties because they are designed for deep cycle and rough service. This article is well worth the time to read it.
If you are considering switching to AGM it is a very different story. With most AGM the plate design are not tailored for specific uses like FLA. Their design provides good performance in both deep cycle and starting applications. Above confirmed by both Trojan and East Penn and is the reason most AGMs spec both AH and CCA/MCA where most FLA will spec one or the other.
When I replaced my 8 FLA 6V GCs I went with 8 AGM 6V GCs as I was refurbing my batty tray and compartment due to corrosion and never wanted to have to do it again. Also the jumper wires fit perfectly by staying w GC.
AGMs are roughly 2X FLA $$ so you need to decide if no maint or no corrosion are important enough to be worth the premium. Many newer higher end autos with more power needs ( auto stop/restart & electronics) have switched to AGM... I have to believe mfg have valid reasons to spend more for a batty!
Many think with 6V battys because you need 2 battys to get 12V you can use half as Many 12V battys... not true unless you want or can get by with half the AH capacity.
A typical 6V GC provides roughly 200 AH at 6V you need two to get 200 AH at 12V
A typical 12V batty provides roughly 100 AH so you need two battys to get 200 AH at 12V.
There is no magic or free lunch with batty difference between 6V & 12V is how the plates are connected... one way provides more AH the other higher V.
Bottom line if staying FLA and you want the same AH capacity you have when battys were new just replace them w the same.
I used mask tape & numbered all wires and drew a diagram w numbers. A photo or a few also worth having. Above let's you get the cables back in the same location when reassembling and cables lengths are not all the same.