Couple thoughts:

Steel vs Aluminium - not important as a criteria for choosing a bike, unless you can get the exact same bike (e.g. the Swift) in two different frame materials. I'm not saying they're exactly the same, but I don't believe that one is all-around better than the other. What I do believe is that bike manufacturers will tout whatever it is they use to influence you to purchase their bike. I've owned both. Different bikes feel different. To try to attribute it to material, seems to me to ignore the many other design aspects contributing to the ride, e.g. wheels, frame geometry, tires...

More from the master, R.I.P.: http://www.sheldonbrown.com/frame-materials.html

Seat post slipping - I had this problem on my Brompton for a while, and it went away after a couple weeks. I believe the best solution is "wait" to see if it gets better. I know no one ever likes that answer for any problem (I don't) but the explanation I've heard, that makes sense to me, is that it takes time for the plastic part inside seatpost tubes to grip the tube properly.