Typically VPNs use SSL between the client and the LAN, anyway. The main difference from a security standpoint is that the VPN approach would leave the communications in the clear once it got to your LAN, while the HTTPS approach would encrypt it all the way to your server. That would only make a difference if there were an eavesdropper on your LAN, between your firewall and your server.
The biggest difference is that HTTPS would probably be considerably more convenient for your clients.