I will send first if trusted or you can sign btc address with btc in it.
PayPal is reversible up to 365 days, even if you send first, there's still a chance of you reversing the payment later.
Btc are not reversible. Once the other party sends their coins and the transaction gets confirmed, there's no chance they can get their coins back, given the situation you were to reverse your part of the exchange.
https://bitcointalk.org/index.php?topic=14632.0Money sent through PayPal and any other money transfer service that accepts credit cards can be easily reversed by the sender, even after several weeks. Therefore, you should only accept PayPal in trades with people you trust very highly.
This is why newbies are not likely to sell their PayPal USD for bitcoins successfully.