Both has its own plus and minus. Most cases in here, bounty started before its fundraising. Kinda rare to find a bounty while fundraising is ongoing. But sometime there's a bounty that finished their private sale. Whichever you choose to, anything could happen at the end of the day. If you're joining a bounty before the fund raising, you won't know if it will succeed or not, and in the middle of fund raising you could see how many investors are in there to invest their money. With all that in mind, you could know how well the projects will be.
It's not actually rare I think, you can track a certain campaign until their fundraising started and have progress. After you see their progress, whether if it is a good start or bad, it will become your go signal to decide to join or not.
If the project is the potential as you want, join earlier, waiting is a waste of time. Also, the first choice is more appropriate because the stakes reserved for jr.member are small. Some may fail, but if you catch one you succeed, the others will be forgotten quickly.
If you are sure and ready to work from the beginning until the end of the project, it is much better. Supporting a project you see potential with from the beginning is better to watch the project closely. Whether it succeeds or not, its result will remain unknown until it ends.