Bitcoin paying signature campaigns take a very few members in their campaign, so you can't join in those campaigns anytime! Everybody loves bitcoin paying signature campaign because most of the BTC paying campaigns are pay bounty rewards on a weekly basis! So, Hunter can sell or trade immediately, no need wait for an exchange. And only lucky bounty hunters can participate in BTC paying signature campaigns nowadays. So, Have a try if you get one.
This is the reason why the BTC campaign is way better. You're sure that you will receive your reward no matter what, unlike other campaigns.
As mentioned above for this you must have a high rank on the forum.