As the title indicates, what is the reason why you are choosing one brand over the other? It seems like 7 to 8 out of 10 builds I see online are using G.Skill memory. The rest are split between Corsair and/or other brands such as Team Group.
Corsair and G.Skill is almost identical in warranty offering.
Is it because G.Skill has more b-die chips in their lineup? Better overall quality/overclockability?

I've used both. I'm currently using corsair memory. I usually go with g.skill though.

GSKILL every time for me. They make awesome memory and it OCs like crazy thanks to their highly binned process. Corsair might have B-die products, but they typically (from our experience on the team) don’t do as well as GSKILL.

I've owned GSKILL kits, my latest one had a RAM failure about a year in but this can happen with anything. I switched to Corsair Dominator Platform about a year and a half ago and it's been rock solid. My experience of course is a sample size of one and I'm fairly confident in recommending either brand to anyone.

I usually use G.Skill, but Corsair is the other premier option. For me price is the weighing factor and that fact that both brands will simply take your word for it if you have tested yourself and do have an issue with your memory. 
 
Customer Service is a big deciding factor for me. G.Skill and Corsair seem to be among the best. Again tho, i've never had an issue yet. Word of mouth is a powerful thing tho.

You can't go wrong with CORSAIR or GSkill.
If Dominator Platinum or Dominator Platinum SE price point is expensive for buyer than I would choose GSkill.
With GSkill in 2/3 situations you will get better performance for lower price.
With CORSAIR more expensive package, pure satisfaction when you buy CMD kit.
Every DIMM in own container, fine sponge to protect them... simply enjoying. I couldn't save cost with DDR4 because I had Dominator Platinum DDR3.
 
But comparing other kits I would rather use GSkill. I have impression that GSkill is more careful with customers and memories they buy and sell and much less price ask. Keep a lot of records, compatibility is very good.
 
Now have some double sided Samsung B-Die, people chase them like crazy.
GSkill sell them I think for cheaper price.
3200MHz frequency I think CORSAIR use them only in Dominator Platinum Special Edition and that's huge overprice.
 

I usually go with G.Skill for my memory.

Not only Corsair and Gskill, I have had good luck with Kingston memory,
Every brand I have owned over the years, have had sticks of memory that failed. And everyone was replaced no issue.
For my 2P and 4P servers, I have had very good luck with Samsung DDR3 EEG-R server memory.

ive always used corsair and crucial (micron) and never had any issues with them, i have some old corsair XMS DDR 400mhz in a old barton core AMD box that still works perfect, its going on 15 years since i bought it.  

I've used both Corsair and G. Skill memory.  Both have worked fine.  I leaned more towards G. Skill in the past because the pricing for similar kits were a bit cheaper.
 
I had one set of G. Skill ram where one stick in the set was dead.  I called G. Skill and they shipped a whole new set to me the same day without question.  I was told to send back the bad set at my leisure and they were sending me a prepaid shipping label.  It's been G. Skill ever since for me.