Its a known fact that race sealed brakes are not designed to handle extensive amounts of road salt. Like any race part they are designed for a specific use...performance. Anything designed for the track will not stand-up to road use for the same amount of time as a road use designed part. Remember race equipent is designed to be rebuilt after a few races as a rule....road stuff is designed to last longer (but often at the detriment of performance)
Basically the issue will be that water/salt will eventually work it way past the seals and you will get corrosion behind them. This in turn will lead to issues on piston travel etc...eg a sticky piston. Anyone who has driven a tuned classic Mini with BBK will have come accross this on the road and race set-ups.
Now I am not saying that the stoptechs will fall apart within 3 weeks (far from it they are a quality bit of kit) but if placed in a side by side test for endurance of the seals/caliper with a road part you would see that the race stoptech is 9 times out of 10 going to have a problem before the road brake. That said there are lots of stoptechs driving around with big milage on them that are fine...its just a warning that out of the 2 choices a road brake will tend to last longer. Same as a non moddified engine will always last slightly longer than a modded on (the question is how much longer
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I have heard several stoptech kits and all of them have a tendancy to squeel a bit when used....more so than a road brake (its all to do with pad compounds and shims and back plates etc etc. If your lucky it will hardly happen but if your not then it can become an irritance. Any race spec brake will have a tendancy to be a bit squeely compared to a road brake, thats the way of life.
Just listen to the BTCC onboard videos when they hit the brakes....! Again the question is how pronounced this becomes on X Y or Z race brake. Some are better than others...the stoptech is one of the better ones...but the AP and brembo kits tend to be even better agin (but then there comes the slight downward turn in performance that these set-up offer due to them not being true race spec brakes)
Its a fine line and there are many choices but for me I think on the MINI the AP kits are not bad. They are cheap, parts are readily available and for 75% of MINI trackday people they will be plenty up to the job. But the porshe brakes offer the big car ventilation/cooling but in a road brake so if your a real heavy booter then they may be a good choice. But I know folks with the stoppies and they love them to the core....just be aware they have thier limitation too in terms of longevity.