As far as venting - both AGM and wet cell batteries produce hydrogen gas when charging. That's why the battery box should be vented. The vent should be at the top of the box because hydrogen is lighter than air.
If you are going to do regular battery maintenance you will get the most bang for your buck with a good quality wet cell like Trojan. If you are going to install them and forget them, then go with AGMs. However, your charger must be setup for AGMs if you go that route.