I don't own a Mini, but I've *almost* bought one twice. On both occasions the reason for not buying came down to a disagreement with the dealer on price. They are tremendously fun little cars - especially the S models with the manual. So long as you're ok with little cars. I am, but some people think they're too claustrophobic. I have read of a few maintenance horror stories with Mini's, but nothing outside of what one can realistically expect from a European car - they're generally fairly pricey to get worked upon.
Regarding octane... Yes, it is a myth that putting 91+ octane in engine designed for 87 results in more power. Your lawn mower isn't going to make any more power on super premium. But, it is NOT a myth when the engine is designed for higher octane gas. If an engine's management system is designed to take advantage of the higher compression ratios possible with higher-octane gas, then you will get more power when running higher octane.
In many cases, even in 'higher octane recommended' cars, you can get away with using lower octane gas because the ECU will adjust engine parameters (timing, ignition, turbo boost/wastegate, etc) to prevent knocking. But you will get less power and possibly less MPG when it does this. See the owners manual to be sure though, as there are a few very high-performance cars out there that 'REQUIRE' premium gas. I doubt a Cooper or CooperS fall into that category.
The last several cars I've owned have all been "premium 90+ octane recommended" cars. So I'm used to the premium (literally, ha!). The price difference between 87 octane 'regular' and 91-93+ 'premium' is almost always $.20/gallon. Whether gas is $2.50/gallon or $5.00/gallon. Which means it costs $2-$3 per tank extra for premium. IMHO, if your budget can't handle the extra couple of bucks per fillup, then you're probably not looking at the right car for you.
