Perhaps start with the one that interests you the most (a group exercise certification is easier to get because it is not as in-depth and the textbooks are shorter). And then, try small-group training as a bridge to the other field. I think NASM has a specialty for small-group training but I don’t know of other certifications specifically for small group. My group-ex certifications covered small group until I decided to focus less on “group” and more on “training.”
As for the time involved, it really depends on which test you take and how good of a test-taker you are. My first group-ex certification, I enrolled in a 56-hour prep course (24 years ago, no longer exists). For personal training, I think I put in about 200 hours.