These are basically two different things, both of which you may want to use to some capacity. Port forwarding allows you to open up a port (in this case port 22) on your outside facing router to allow traffic to a certain computer on your subnet (the computer you want to SSH into). This essentially opens up your computer to the outside world so you can log into it from outside your local intranet.
The dynDNS portion is what would allow you to point a domain name (EX www.mycomputer.com) to the IP address that your ISP assigned to you. The reason this is so helpful is that most residential ISPs will use a non-static IP address (meaning that it changes every so often) and this would require you to keep track of the IP as it changes> dynDNS allows you to use the domain name and have it automagically resolve to whatever IP you have at the time.
So in short; you will most likely want to use both in order to SSH into your machine from outside your local intranet.
As far as security is concerned you should be ok on a properly patched/configured system. Of course there is always the potential for attack when you open a computer to the internet but it is regularly done without much problem.
The only thing you are likely to see is scripts attempting to break your password but that is why you use public keys to authenticate ;)