I'm basically new to turbos, all I know so far concerned normally aspirated motors.

What differences are there between S4 and S5 turbo II's?

 

It's been covered quite often, but a quick list:
S5 has different:
rotors
housings
injectors
ecu
elevtrical system
cruise control
front cover
omp
turbo
manifold
no egr
tps
supension parts
wheels
lsd
front & rear bumpers
side skirts
body molding
taillights
fog lights
conso;e
package self
alt
radiator
exhaust, at least the part numbers are different
dash gauges
steering wheel
interior, black leather avail
rear hatch light

 

lsd? you mean the s4 doesn't have lsd?

 

Both series of turbo models have a limited-slip differential. Just the method employed in each is different. IIRC the S4 turbo has a clutch type LSD and the S5 turbo has viscous type LSD. Generally, the clutch type wears out quicker, but is more effective than the viscous.