I got certified in both and I would recommend that. The reason is you end up with a little more training by getting both certs. It really takes repetition to be comfortable with deeper dives, at least for me anyway. If I have been out of the water for some time, I won't go on a deep dive until I have a couple dives that are within the open water regulations or close too. That is just my preference. All my training and certification has been cold water and I am continuously learning every dive.