Firstly, you should aim for the knee with the weight on it, for someone who doesnt do MAs that would be the front knee nearly all of the time, which is very convenient. The more weight on the knee the easier it will be to break and the harder it will be to defend, since it would take longer for the opponent to shift his weight.
If he knows wing chun and has the weight on the rear knee then i would not advise you to go for the rear leg because it would leave you open for attack since you will be offbalance too near to the opponent for comfort, so what i would do is go for the knee of the front leg which wont break but will hurt, and then slide down the shin and land on his foot trapping it and then go in with hand attacks.
If you go for the inside of the knee then it should result in a break if done properly, if you go for the outside it may result in a break but it may also just make the opponent twist down onto the floor, in which case your foot should keep contact with the back of the knee and then jump and land on the exposed ankle with your other foot, breaking that instead.
I m not sure if what i am saying makes sense.