Wondering which of fsl and AS has a320 engine sounds closer to real counterpart. To my ears comparing in videos and actual travelling, the AS a320 engine sounds appear more realistic except for a little lack of thickness to them while fsl a320 engine sounds appear bit too thick and very up close. Any idea why fsl has sounds that way and do others find it more realistic?

It really depends on your sound card and also the position of the original recording. For example if you sit in row seven or less the LP compressor noise is extremely loud as is the aerodynamic fuselage noise. Sit further back and the LP compressor is much less but there is more eflux noise. So in the end it's swings and roundabouts.

engine (only) sounds ....from the flightdeck, the cabin, the fuel truck, the tower, the terminal ???????

comparing with engine sounds from sitting few seats behind the cockpit, and videos from front seats and cockpit at takeoff. The fsl is bit synthetic sounding and fan buzz is much dominating and rigid. In AS one the compressor whine is more natural sounding and more prominent as appears in the real bus, the humming while taxiing is also well reproduced.