[Al Skierkiewicz]

Sam,
There is relatively little difference in the two for impact resistance. The gel uses a cardboard separator and the AGM uses the glass mat. There are tradeoffs in using either technology but the AGM has proven useful for slightly better energy density in our application for the same size case. Although I have no data to prove my theory, I believe that certain impacts can lead to plates punching through the glass mat and shorting a cell. There just seems to be more shorted cell failures with the AGM over the gel cell in our use. MK did some forensic analysis of dead batteries over the past few years. They found nothing significant.
I agree that the CBA battery analyzers from West Mountain Radio give a good indication of battery health. They are repeatable and allow comparison (overlay graphs) over several tests. As pointed out, they can be set to current draws that are far less than our intermittent loads. However, a 7.5 amp draw from an 18 AH battery is sufficient to tell the health of the battery and calculate amp hour ratings that match the manufacturers spec sheets.
Please be careful when listening to anecdotal info on battery use and performance. Some teams use high current chargers especially when not in competition. Team batteries are stored in unheated areas in winter or uncooled areas in summer and may be left uncharged for months at a time between seasons. All of these lead to early battery failure. Above all realize that these batteries under normal conditions only have a 400 charge/discharge life. Under our use of high discharge rates, charging while warm and the other conditions described above, the battery life is significantly less.