This is a loaded question, and the answer is --- it depends. Our experience in the solar and wind energy field leads us to believe that conventional lead/acid batteries provide more bang for the buck than AGM (absorbed glass mat) or gel. While it's true that AGM batts can have slightly better charge/discharge characteristics, and don't require adding water, we've found them to be a LOT more finicky than conventional wet cells. They seem to require very specific charge voltages and don't seem to hold up as well when deep cycled. They also cost several times as much per A/H (amp/hour) as conventional batteries.
Conventional batts seem to be more forgiving of overloads, improper charge conditions, and deep discharges, HOWEVER, they do require routine checking and filling with water. Soooo---if you have a well regulated system that you don't deep cycle, and you want to pay four times the price for not having to add water---buy the AGM batteries. But if you want max power per dollar and don't mind checking on them occasionally, save a few megabucks and go with conventional. I would recommend a good quality unit, such as Trojan or Deka---don't try to save pennies when it may cost you dearly later!
For a MUCH more detailed discussion go to some of the solar battery sites and read through the specs and discussions. . .
Grover--- Resident prognosticater. If you need specifics, contact me at email@example.com.