Which is better, gel batteries or agm? What is the life span of each and which one will accept faster charge without damaging the battery? Are regular lead acid batteries acceptable?
All 3 types are lead-acid and will do the job, some for less cost, some need battery-temperature sensing chargers. Gel will last longer with multiple deeper discharges, agm will charge (faster) with a higher voltage. Gel & agm are sealed under pressure and will be damaged if charged at a too-high voltage, which could cause pressure release to vent and permanently ruin battery.
Probably most of the group have 2 or 3 8D size house gel, but agm is also popular & flooded-regular is least popular. Start banks are often 2 or 3 1000 cranking amp red top agm, but need to have a Trik-L-Start-type $50 'charger' connected.
Your charger must be properly capable and setup for each type in order to extend the life of the battery. Most of us are probably replacing batteries at about 10+ years of age, whether they need it or not.
Granpa
Perhaps the only topic more discussed on this form than batteries are tires.
Grab a favorite beverage, take your laptop, go to the forum home page and use the search function for batteries. Lots of great info, some not so great, and a lot of opinions on your question.
Consensus - Gel and AGM have their supporters, lead/acid not so much.
Agm are good for 500 50% discharges. 250 amp hour capacity.
Gels are 1000 50% discharges. 225 amp.hours capacity.
Calcium grid in the gels. .
See east Penns website or mk batteries.
Foretravel has used gels for oem builds forever.