HRMINI

Well, this is a something I have been debating as well.  I have experiance with both and it depends on what you want out your Mini.  First thing to think about is if you are doing the rear at the same time.  If not I would stick with the rubber cones as coils come in a set of 4.  If you plan on doing the whole car and want longer life go with the coils.

Also, look at adjustable trumpets if you go with coils to aid in the tuneability.  Rubber cones are great because of the simplicity and are more progressive than the coils.  Coils are nice because they last so much longer and are a bit more linear in their travel which I feel aids hard cornering a bit.  Plus with coils you can pick from 3 spring rates, street (soft), fast road/rally (medium and my pick), or Competition (hard).  Rubber cones really only come in qualities, bad or good (cheap or expensive depending on your view).  You can get competition cones but they will rattle your teeth out.

Don't forget when you have the suspension broke down to replace your knuckles and knuckle cups and re-grease. 

BTW, replacing the rear springs is a ton easier than the front so you can easily do them yourself, no special tools needed.  The front requires a spring compressor tool.  Hope this helps.

Shawn