Because Painkiller. Literally begins and ends with that.
I own two pair of genes with them (2 seras and 2 dieters) and economically speaking, especially after Cylinder, she is A LOT "cheaper", because it will trivialize leveling so you barely need to pot and you don't need Dieter extra damage because your damage is already pretty god damn high.
Going beyond leveling, imo they both do different things: Dieter makes bosses faster, Sera makes instances easier. Stuff like Malangdo, Charleston and Sarah and Fenrir are a pain because stuff hits like a freight truck, you have relatively low HP and leeching is very poor on the class, so you'll have to pot quite a bit, but if you use Sera that suddenly becomes a joke. Yeah she is worst to someone like Charleston herself but the supplies you save from not having to pot like crazy definetly makes up for it.
Dieter is better for BBs, camping MVPs, ET and Central Laboratory, where the main gig about those is "Kill the boss".
Sera is better Hazy Forest, Charleston, Devil's Tower and Malangdo Sewers, where is more about minimizing the costs to increase the profit.
In general, Sera is probably the best for solo players, but if you're doing parties with guild or w/e I say go Dieter because it will maximize your damage and you'll usually have an AB to "get you". Not that you can't use Dieter solo (on the contrary) or vice-versa, but that's the impression I got overall.
Edited by LunaMoonBunny, 24 March 2017 - 05:52 AM.