Aluminium is cheaper and sometimes lighter.
Carbon is usually expensive and lighter. Carbon soaks up vibration and bumps better than aluminium.
don't obsess over the material. Test ride some bikes and buy the one that feels right.
Close clearances are pointless, IMO.
I suspect you are associating them with snappy handling that feels fast.
Hi mrcharly. I personally have never ridden a Carbon frame - and my pension would not support such frivolous fashion.
Carbon is known for being a severe ride and very little kindly to the skeleton as a quality steel frame is.
Carbon is also known for sudden and inexplicable failures as it is also known for crushing if one is a little heavy-handed with Spanners and or Allen Keys.
Whether it be 531 single or double butted or even Columbus tubing - there is a comfort level that really does take some beating.
Rider weight and Saddle choice has a great-deal to do with rider comfort and as does bar/stem choice and the appropriate wrappings of bar-tape.
My road bike is a Steve Goff Columbus tubed - Designer Select - three different Columbus tubes chosen by Steve Goff for their particular attributes.
I have just built a Bianchi Via Nirone 7 Alu Carbon (Carbon Forks and Carbon Seat Stays) as a Flat-bar Road Hybrid on Campagnolo Veloce levers.
At almost 72yrs young the blend of 7000 Alloy and very little carbon is about as much as my old bones will tolerate on our crappy roads and with the comfort of the Flat-bars for a leisurely and upright ride.