Forgive me, alienmeister, but are you joking? If you want to celebrate your 16th birthday in one piece I would most definitely reconsider your shortlist. The accident rate amongst novice drivers is bad enough already.
Neither of these cars is at all suitable for learning to drive (I am assuming you haven't a licence yet, and even if you have learned car control off road, that is not the main issue). A car is a potentially lethal weapon (and not constitutionally guaranteed), and these two, esp the M3, are nukes in inexperienced hands. And an M3 if not properly (and expensively) maintained is worse still. Sure, it has loads of clever gizmos to keep you on the road, but what if one fails?
Think too of the damage any learner will do to a car like this. An M3 is like a thoroughbred racehorse (think Kentucky Derby winner), with a delicate 'mouth' (controls) and highly strung temperament, bred to win races against the best. Do you honestly think that's the mount for learning to ride/drive?
Also a 'racehorse' M3 is speedy, which means dangerous situations appear and develop much faster. It takes the average driver 0.8 secs to react and start to apply the brakes (how far has he travelled in that time?) and even the best are around 0.5 secs, but what distinguishes an experienced driver is being able to spot the telltale danger signs (in the environment and themselves/their companions) much earlier and assess their significance, so he/she will react earlier.
Buy something 'sensible' and in good condition that you can afford to maintain properly, get a few thousand accident free miles under your ass, a couple of years learning car control and roadcraft so you learn to spot and avoid accidents before they happen, at normal speeds, and then ask again. They say that you only start to learn to drive once you have passed your test. With experience you will actually appreciate what these cars have to offer, as well as being able to drive them competently. When you have built some experience, invest in the best performance-enhancing accessory there is - an advanced road driving course.
You owe it to yourself, your family, your friends and fellow drivers. Have you ever seen someone in a bad smash? What if it were someone you care about, due to your inexperience or incompetence? You would be haunted for life.
PLEASE RETHINK.
Rant over. Yes, I'm an old misery, but I love driving passionately, and I hope you wil learn to as well. Most important, I'd like you to survive to my age someday, too! (Awful prospect, eh?)
To finally address your original question: driving is all about awareness and judgement. If you ask a website of M3 freaks, just what sort of answer are you expecting? Hopefully you have posted on a Nissan website too? Even so, will the answers really help you decide? Or are you just seeking views? If so, you need to use more open questioning - like 'why?'. Also, there might be other cars to consider, or have you based you search solely on performance per $? (Don't get me wrong, both are fine cars - indeed, the Nissan was the alternative when I bought my CS, which I believe is called Competion or Club Sport in the US.)
With regard to breakage, almost all the UK M3 owners I know have some sort of extended warranty. The cars are now much more reliable (check yours had the two recalls for bottom end/oil pump - if not, walk away) but even so, if they do let go it can be very expensive indeed - think US$20 thousand for a new engine in the UK, and BMW labour costs are high (see ase2dais post on thread 'Need some input quickly'). If you must have one or other, buy the cheaper to maintain newer Nissan now, and look forward to the 'real car' M3 when you can afford to own/maintain a good one. (Between ourselves, the M3 can be a pig to drive for a few blocks when cold.)
Good luck with whatever you decide - if you do get an M3 get SMG, it's truly awesome (see thread re racing an 'auto'). Come to the UK if you want a demo!
