A girl on my train yesterday got fined for not having a concession card. They're really locking down on it now.
First few months of uni, i thought i didn't need one either. Almost got fined...*twice*.
Luckily, i was able to talk my way out of it but it's not really worth risking it. The fine is a few hundred bucks. The concession card (the one with your photo etc) is only probably only like 10 bucks. It makes sense to not get caught and hassled and it makes good economic sense too.
There's no real reason to not get it, even if you're only going on buses where your chances of getting caught are pretty small compared to the train. It's one of the easiest fines they can give out too.
Go here and click "compare pass to money", it'll tell you which one is better in light of how many days you go in and things like that.
As b^3 said, anything under 3 days a week, you should get myki money. Anything over 4 days or more a week at uni, it's worth to get the myki pass. Also note the pass lasts for the entire month, so, if you decide to go out on the weekend and use the train, it won't cost you extra. If you're using myki money, it will. So, factor that in too.