The advantage to coming on as an FO is that you can come on with less than the 1200 hours that 135 requires for a captain position. As mentioned above, you then can transition to a captain position (most likely a BE99) when you get your hours AND generally after you have spent a year in the right seat of the Bro to cover you training commitment.

Yes, you will likely spend at least a year as an FO before transitioning. So if you are over or pretty close to 1200 hours (and a way to get there) you might be better off getting your 1200 and hire on as a captain in the PA31.