I'd say coils if you're doing this to improve handling as well as looks mate. Most brands are almost infinately adjustable so they'll only be as firm as you want them to be.

I lowered mine on springs at first and the car sat too high at the front. Also, lowering the car on springs without upgrading the shocks is a big mistake. Better off spending a little more, getting springs and shocks that are designed to work together then once you've sorted your perfect ride height, get a proper setup done somewhere like Dreadnought or Andrew Gallacher.

I've got KW V1's as my mini will never be on track and they're phenomenal for road use. Ride quality, looks and handling have all been improved drastically