Sessions table myisam or innodb

Hi;

rt-3.6.3
apache2
mysql5.1

For better performance should the sessions table be myisam or innondb??
had a look in etc/schema.mysql and there is no engine type definition
for sessions
currently my sessions table is myisam, every now and then rt freezes due
to user lock on the session table. To keep it small, I have a script
that delete expired sessions every 6 hours. however I am wondering if
this is causing another problem with concurrent inserts due to deleted
rows in the middle of the table ?/ any suggestion.

Regards
Roy