Left gacha usually features all the new stuff. The right gacha varies. There's revival gacha for older units, farewell gachas which have all units at level 40 and is pretty much a farewell for units going to revival gacha, and pick-up summons where only specific classes are featured. I guess you could tell which is which since revival gacha only has the old units that you know and love from Nutaku Aigis, farewell has the new units and there should be a number 40 on the banner, and pick-up has a mix of old and new units.

Veterans usually recommend revival gacha over current gacha for newbies is bcuz the new stuff has so many new classes (we've been getting a whole lot of new classes lately) that it is kinda hard for them to use the units efficiently. Plus we're getting more hybrids now. Heck, we're getting a new oni-class after this week's maintenance which is pretty much a physical AoE bandit.