Hate to go against the forum, but I'd personally get the mini as much as I like the 99-00 Si in Electron blue. Power wise and suspension wise the Cooper S owns the SI in stock form.

Also illusioncivic commented on how if you turboed or SC'ed the SI which comes in N/A form that it will last longer or will be better in the long run. I personally dont have any experience with the Mini's but logic brings me to the conclusion that the SI will NOT last longer in the long run. For example, most engines that come SC'ed or Turbo'ed from factory generally tend to be better reinforced, built ALOT stronger, and are able to handle more power than the N/A. Take the eclipses and supras as an example. No one in their right mind would try to boost a GS when you can easily get a GST... as well as a trying to boost a regular Supra. On another note, Honda engines were never meant to be boosted as they blow up under low boost--i.e. try boosting a stock B16A with upgraded fuel management. You wont get very far with boost safely. You could probably boost up to 10-12psi safely. Even then its pushing the limits of that engine, and also it wont last that long. (And yes, some people have pushed there engines up to 15psi and more in stock form, but I considered those engines "FREAKS" and most of them wont be reliable daily drivers!)

I hope I made some sense as I am going on very little sleep at the moment.

Besides that technical shiet... I think the Cooper S is more of a "babe" magnet than the SI (as there are one too many ricers with the SI). =)