That depends on your platform, your EDI translator and your ERP, and last of all how much money you want to spend, IMO.
Do you need another PC for the AS2 software, do you need a firewall/DMZ? How will you integrate the software to the EDI system and then get the data to/from your application?
I know this doesn't directly answer your question, but what is 'best' to me will not necessarily work under your situation.
Good luck.
Bob