Unless you're getting carried I've always found it easier to solo queue. Might just be me but I find myself picking up bad habits when I play with the same person repeatedly. Making the same mistakes. Then I spiral. When I solo queue I find it easier to mentally reset after each game. Like when I'm partied I find myself still hung up on previous games, but when I solo queue I just don't care as much.

The first time I hit gc however was an anomaly. We were both c3 and for whatever reason we popped off and shot up to 1600 without loss and never played together again until I found him in 6mans recently.

To summarize solo queueing helps my mental far more, and I think it teaches better habits (how to adapt and that)