Some car will only accept immo3 clocks 52 plate onwards,older cars can have either immo2 or immo3.The only difference between them is that they can have miles till empty and illuminated needles activated.Both these functions are overated imo,miles till empty only makes you sad when you see how little petrol you have left and if it's dark enought to see the needles illuminate,it's time to turn your lights on anyway.
I'd have immo2 and pay less,but that's just my opinion.