I'm not aware of any significant new bugs. If you're using 2.5 and you're happy with that, then feel free to stay on it for a while longer.
At some point you may decide to move forward based on either new features or support of the old software being discontinued. AnyConnect 2.5 is no longer sold and there will be no new bug fixes released after next summer. But, for now, 2.5 is still a supported product. (Reference)
3.1 has a lot of new features and presents an updated and more attractive user interface as well. Key new features include:
- Next Generation Encryption-Suite B: Cisco has raised the global standard with next generation encryption, NSA's Suite B Cryptography utilizing IKEv2 / IPsec to ensure highly sensitive connections while still offering an optimal end-user experience.
- Mobile Broadband Support: Network Access Manager's connectivity options have been expanded to support mobile broadband (3G / 4G) connectivity. As with Ethernet (802.3) and WiFi (802.11) connectivity, the AnyConnect Network Access Manager continues to support one connection at a time and automatically switches to Ethernet when available.
- Global IPv6 Support: To ensure security for the next wave of IP addresses, the new IPv6 features include the ability to tunnel over an IPv6 public network to an IPv6 security gateway. IPv6 traffic can be tunneled over the connection for Secure Sockets Layer (SSL), including Transport Layer Security and Datagram Transport Layer Security (TLS/DTLS) connections, but not for IPsec with Internet Key Exchange version 2 (IPsec/IKEv2) in this release.
- Differentiated Device Access (EAP-Chaining): As personal devices proliferate onto the corporate network, the need to distinguish between corporate and personal devices will be a security imperative. Cisco is again leading the way to help IT professionals differentiate network access authenticating both the device and the user in a single transition.
- Enterprise Connection Enforcement: Just as Internet enabled devices are coming in all shapes and sizes, so is the wave of available networks in common areas from airports to cafes, even extending to corporate sites in urban centers. Enterprise Connection Enforcement helps IT Professionals enforce IT policy on the endpoint preventing these connections when the corporate network is in range.
- Enhanced end-user experience: The industry-award AnyConnect 3.0 user experience has a fresh new look and this experience is also available for OS X.
