Here is some of my cursory auxiliary battery info:
Optima: conservative rating 65AH (probably more like 80AH) 10x7/8x9", $170,
1yr warr. (Note:this battery is rated at 775CCA but actually tested to
1275CCA)

Exide AGM: same technology as Optima, rating 50AH (probably conservative) 10
3/16x6 13/16x8 1/8, $102, 4yr warr. (Note: good value for AGM
technology,great warranty for deepcycle battery)

Optima 6V: rating 80AH @12V(per pair) size was n/a but basically 1/2 of a 12V
Optima, $100each, 1yr warr. (Note: $$$but increased mounting option and more
AH)

Trojan T-105 6V: rating 225AH@12V(per pair),10 3/8x7 1/8x7
3/16"(ea),$63.20ea, 1 yr warr.
(Note: needs to be enclosed,vented and probably only will fit under rear
seat. Big,heavy, and most bang for the buck)

GNB1244 (gel cell): rating 44AH @12V, 7 3/4x6 3/4x6/7/16", $90ea, 1yr warr

U1LDC (gel cell): rating 33AH@12V, 8 3/4x5 1/4x7 1/4", $55ea,1yr warr.(Note:
good value in pairs for  66AH rating, smaller size for more mounting options)

This info gathered by phone locally YMMV. I heard the Optima patent has
expired which is why Exide and others are now free to market their own
essentially the same version.
Hope this is useful.

Ken Hunter
82 Westy 1.9D
84 Jetta 1.5D
89 Mitsubishi van