I assume you are in N America. Here in the UK starter batteries generally are given an Ah rating and usually the newer ones also have cranking amps and sometimes CCA.
I think generally in N America the Ah figure is not generally given for starter batteries.
No form of starter battery is really going to perform well in place of a deep cycle battery. A starter battery will actually give its Ah rating ( if you know how it was rated) but it won't stand many cycles to 80% discharge.
In general the 6v batteries tend to be higher capacity than 12v ones, it's just a physical issue that large capacity batteries are heavy, the larger ones are built for 6v to keep the weight manageable and you use 2 in series for 12v.
In general starter batteries are a poor choice and unless there is some obvious reason to use them then it doesn't seem a good idea. If they are cheap enough then it may be cost effective to accept a short life. I have also to admit that the best quality starter battery may actually give better results than a crap deep cycle one of the same rating. Often cost and weight is a better indicator than the crap on the label.
Flux
