Greetings Mate,
Well, from your description the obvious answer depends on how much security you want or how much configuration you want to do?
If you want to have simpler configuration, forwarding a port in firewall would be a good option, but then you will have to rely on Terminal Server's encryption for security. And if there is a vulnerability in Terminal Server (the chances of which are more) you might be in trouble.
On the other hand if you want to use Cisco VPN which I believe is more secure, you will need to invest a bit more time in configuring Cisco VPN. But obviously this will provide more granular control over the user's connection.
Infact, you can configure both Cisco VPN and Terminal Services Encryption if you are a seriously paranoid about the security...But in the end it all depends on what your requirements are.
I would prefer Cisco VPN in most cases, unless I dont have much time or I want to keep things simpler.
* Please rate if it helps