IMHO, I am a big cheerleader for the Citi DC card. I have dealt with their customer service a couple of times and they went above and beyond to help. Not only did they make me feel like a valued cardholder (which some companies could really learn from), but for an inconvenience, they issued me a credit into my rewards for the hassle I went through which was no fault of their own. I didn't even ask for it. That spoke volumes to me and it's one reason why it's my card to use when my purchases aren't directed to a specific rewards target.
I have dealt with Synchrony customer service and they were average. Nothing special and nothing horrible. I will say, I really do not like their website and the setup of it. It feels like I'm going back in time to when Al Gore invented the internet. 
It's not the easiest to navigate. I do keep my card with them for the CL it has. I guess one plus is that they do offer the Vantage 4.0 score. While we all know it's a FAKO, it's nice to see that one, too. I have FICOs from almost all my other cards.
With Citi, I have been able to progress pretty quickly with my credit limits. I started with a SL of $3100 and I'm now up to $9600. I also like that there's not a HP when requesting. It mentions it before I ever hit the button.
I also really like the Citi app. It's very easy to use and does everything I need it to. Given I'm not always on my computer, that makes a difference.
One thing that I will mention, I'm not sure if the PP Synchrony has the World Elite status for their MC, but Citi does. That can provide some nice little perks along the way, too.
All in all, I love my Citi DC card. I look forward to many happy years ahead!
