When I used to be active in gold market in september/october I had multiple times more trades with 07 gp, so I think 07 is better investment than RS3 GP
From what I've seen RS3 GP has been rising a bit the past few months. Unless you've got the money to start up your own shop it probably won't too profitable for you. And if you have no previous experience marketing RS Gold or just marketing in general I'd highly suggest not sinking too much into it. It would be good experience though. I'd keep my options open to both RS3 and 07