GGeorge,
Assuming that the bank is just the two 6V in series and the surviving is actually a year old, I would just replace the damaged one. Batteries live and die by discharge cycles. The two will not be far enough apart to matter.
Now, if this were a pair of 12v in parallel, that would be very different.
As to AGM, I love them, but I don't think that they are cost effective in most applications. Get prices and then decide. Yes, they can be discharged deeper without damage and be charged faster and generally last twice as long in similar service, but you will have to decide if any of that matters enough to you and how you would use them.
Before the depression, I did a lot of boat work and installed a lot of AGMs. Owners never regretted it that I heard.
Matt