Should I wait

Hi guys, I’m 19 and planning to get a bike fairly soon. I was looking at getting my cbt up to a2 done and getting a ninja 400. However part of me wants to get my cbt done and get a 125 to ride for a few months to get used to riding since I’ve never ridden before, I’ve just done a fair bit of mountain biking. I was wondering if you guys think I should just wait for the 400 or not.