Hi edijos,
With SSL, if you are only validating the server you won't need to have the server's certificate stored in your system, the certificate will be send to you when the session is being stablished. If the server requieres client certificate validation, then you will need to have a valid certificate in your system.
Using SSL you only need to have your certificate (if needed, otherwise you won't need a certificate at all). So, you are right when you say you don't need to have all those certificates in your system.
SSL encrypts the session, whilst AS2 security (PKI framework) encrypts the payload (content or data) you are sending. They both offer enough security regarding the privacy, authenticity and integrity of the connection.
One advantage SSL offers over signed and encrypted messages is the authentication of the connection itself, so, the system resources are available only after authentication, if you are not authenticated, you can not go on with the communication.
Using signing and encryption, the system still needs to process what is getting over the communication channel, even if you don't have a trading relationship with the sender.
HTTPS helps the system from getting overloaded with unauthorized traffic(spam or denial of service attack) but it does take more bandwidth and processing cycles.
If you use HTTPS(SSL), you should not use signing and encryption and vice versa, they are kind of redundant, and using both will result in an unnecessary use or resources.
HTTP used with signature and encryption usually provides better security and is less intensive with your system resources.
HTTPS is a better option if you are concerned with spam.
