it all depends on which one you'd rather learn first. Neither will effect the other. I love stoppies. They're addicting and one of the most awesome looking stunts there is. Honestly I'd learn both at the same time. In my experience, if you try to learn one trick, for an extended amount of time, you end up losing your skills in the other. So, warm up with your sit downs, try a stoppie, or 5, then try a standup or two, then do a sitdown, then a stoppie... this way you get more than one trick on lock at a time and your back of tricks grows faster and is more solid. Good luck and be safe!