GC vs AGM are just about the same.
Cost: AGM does cost more but last longer, my Lifelines are 9 years old. For a few years use and sell the rig then GCs are cheaper.
Same amount of lead and weight.
Charge the same, slight voltage difference.
Vibration not a issue.
AGMs also don't vent or use water. So less corossion and maintenance than GCs. Some have put AGMs inside.
OK if a AGM does vent then it has a dead cell or was over charged and in any event it's now toast.
For reference: My Jeep has a AGM under the passenger seat and the carpeted cover is NOT sealed. The vents are enclosed in a pastic shell that that is part of the manufacturing process and covers the vents only. It has a 1/4" tube to vent under the car. It's 6 years old.