In all honesty, save your money.
They used to be an insurance requirement on my previous Imprezas - which were a much higher profile target for professional thieves than a MINI (especially ones looking to use it on a job - can't see too many real world bank robbers fitting in a MINI
). Even then, what they saved me in cash on the insurance, they cost me in the monitoring subscription. Ended up changing my insurance company in the end to one that quoted me less and didn't insist on a tracker.
The last one I had (a Tracker Horizon) chewed the battery to the extent that if you left the car for a couple of weeks without driving it, the battery would be flat and the tracker next to useless anyway. Case in point, I was working on a contract in South Africa when I got an alert (because my battery had gone flat in the Heathrow car park). When asked if I knew if the vehicle was secure, I explained that I was 6000 miles away and would be for the next 6 weeks so I asked them if they could verify the cars position for me. I was told that they couldn't because the tracker wasn't responding and they couldn't send someone down to Heathrow to check if it was still there. Was glad I stumped up the £135 a year for that, I can assure you.
The bottom line is, if a thief wants your car, they'll have it whether it has a tracker or not. Especially as most cars are stolen by nicking the keys these days which won't activate the tracker until you find out the car has been nicked and activate it yourself. By which time, they could easily have removed or disabled the tracker - took me about 10 minutes to remove mine after it was no longer an insurance requirement.
As much as we like our MINIs, I really don't think that there's any need to worry yourself unduly that they are top of the car thieves list of cars to steal - if they were, our insurance premiums would be astronimic and they're not - mine is about 1/2 that of an Impreza which tells you something, I think.
However, if it makes you feel better, then fit one but don't kid yourself that your saving money or really making your car that much more secure. Bottom line is that it's really just a means of extracting money... suspicious that UK Insurance Ltd (Direct Line etc) happen to own Tracker and then insist on you having a tracking device fitted on certain cars, don't you think?