Cisco WLC 3504 recommended firmware

Hello guys,

I'm designing the network for an event where about 300 ppl are going to use my wifi through a captive portal.

I have a couple of brand new 3504 in sso. They are not going to do anything fancy, but which version would you recommend? There are so much firmware available on cisco.com i'm a bit confused...

Thanks !

Edit: i have 25 3700e AP ;)