Welcome to the forum Lawrence.
Both engines seem pretty bulletproof, it only tends to be ancillaries like starter motors, alternators, etc. that die. Although the 2006 facelifts have an electric water pump that will die at some point causing overheating within a mile, and new ones cost £250 to £300, you'll probably only need to replace it once.
If the 2006 2.5Si is a Sport model it will have the M-Sport seats which are very desirable but are very rare on pre-facelift models and tend to sell for £600/700 or so on here.
All 3 litres have electric seats with driver's memory as well as headlight washers although they are a PITA as they often leak!
Spec is a big factor as nobody seems to want an E85 without heated seats, and Xenons are sought after.
Beyond that it comes down to mileage, condition, service history and price.
If you post links to adverts on here you'll get some helpful advice.