SimpleServer vs HL7Server

I am new to HAPI and would like to know if there are any pros or cons to using SimpleServer as opposed to using the HL7Server? 

I would prefer to use the SimpleServer since I can register an Application with the SimpleServer and I do not see a way of doing this with the HL7Server.  If I am wrong please let me know. 

Also I may not be interested in having all messages received saved to SafeStorage, so if I did use the HL7Server class then am I able to pass Null for the SafeStorage parameter in the HL7Server constructor?

I just want to make the right decision and not use SimpleServer if it is not really a full blown robust server as HL7Server may be?  (Maybe its just the simple part of the SimpleServer name that is throwing me off)

Thanks for any help in advance!