I'd get the M3. No question which is a more competent driving machine.
I suppose you'd have to go with your mum's final decision as she is the key to the deal.... either that or just spend your 'half' on a car (E36M3^) you don't have to share.
If I were in your position, and 3 years ago I was, I'd put the money into a house and move out.
<span style='color:blue'>"So my BM makes me a flash bastard? Then I guess you're just keeping it real from the seat of your nova?"</span>
<span style='color:gray'>"...and the home of the brave..... is bravey 130bhp from a 7 litre engine? 100bhp/litre or don't bother building it!"</span>
<span style='color:red'>"Aaahh the UK; where every road is jammed, under construction, poorly maintained and unfairly policed, at least we don't get over-taxed for it........ HA HA HA!"</span>