trying ... to compare apples with apples.....
standard 4.8 auto vs standard 4.5 auto:
Fuel consumption: Very little difference if driven in the same fashion
Power: 4.8 = better power delivery due to engine power and gearbox ~196 vs 156 kw + 5 vs 4 speed auto
Interior: 4.8 /2005 and later updated more "modern interior" ; The interior of the 4.5 will age better due to more durable material used.
Reliability: Both vehicle are very reliable provided that they are well and regularly maintained. The 4.5 might have the edge in this department due to the fact that it has less electronics.
Cost: As per Tinus / Overall the 4.5 will be the more cost effective vehicle option i.e. purchasing cost, insurance, maintenance.
General: The majority of the 4.5 appear to have manual 5 speed boxes whereas on the 4.8 you will get them mostly in the 5 speed auto version (manuals are rare)