Most people do, as you say start out on the 737, but still some on the MD80. Last class I heard of was 8 on the Boeing and 2 on the MD. Now that might all change this coming summer and fall, when a lot of people will be going onto the new Airbuses. A lot of those will be coming from the MD, as most newhires have been assigned the 737, and therefore are "stuck" on that one for 3 years.
Base?? Well I´m danish, so I prefer CPH
..
Depends on where you would like to live!!
Operational differences; not many, but european flights out of CPH are somewhat shorter, something I like.. STO-MXP is to me the longhaul that I dread...
Salary a little different, in that currency exchange rates make (at THIS time) salary at CPH a little higher. (But then you pay higher taxes here)