Very simple to answer , choosing a short period bounty is always better to do , bcoz after that bounty we can choose new project to do and can allocate our time to most of the projects and hold more coins in less quantity rather than holding only one coin in more quantity .
however if budget of bounty with longer period is much more in proportion than we can decide to choose longer one , and it also depend on project potential . joining a bright project even with longer period is also justified .