Gemini is ok, but less volume than GDAX and I'm not sure they have a free type of orders. Also Gemini's interface is personally less interesting for day trading. But to that point GDAX's graphing interface is a bit odd and represents data differently than say trading view.

Trading view is by far the best to check out a coin, see its indicators etc imo.

Still I think it's good to diversify your funds over various exchanges, just in case of a hack.

Kraken did a massive upgrade so it doesn't have issues like it used to. People haven't come back yet however, and the spread is pretty bad. It's the only exchange I've found to support margin trading and shorting for US customers which is why I went back.