Both tools are now at version 14.01.0180.003.
Jetstress can be used to check performance and stability of storage under load. It simulates Exchange I/O behaviour for Exchange 2003, 2007 or 2010 in order to verify dimensioning and check if storage meets performance expectations for a certain number of users with a certain I/O profile.
LoadGen can be used to simulate MAPI, OWA (2007, 2010 or 2010SP1), EAS, IMAP, POP and SMTP client load against Exchange 2007 or 2010 servers. LoadGen will use real clients to simulate the load and, like Jetstress, LoadGen can be used to verify dimensioning and check if the design meets performance expectations.
Unlike Jetstress, which can be used to verify things from a database perspective, LoadGen can be used to verify things from a client (handling) perspective.
Note: If you used the beta version of JetStress you should recreate the databases for the RTM version as Doug Gowans found out.