I am in this sort of position as well(interviewing to become a GSA). Target seems to take forever to go through the interview processes though, so it can be rather frustrating. Also, I sat down with my STL to go over interview stuff and it seemed to me like it went poorly (mostly because I was so nervous talking to my STL, and I was exhausted because I slept only 5 hours) but my ETL-GE said that it had went well. Anyways, I would say you should take the GSA position because it is a position where you WILL move up quickly if you are successful and show willingness to move to the next level (Assuming positions are opening up at a steady pace of course).