4.8is. Yummy.
I'm with you. I do not like to go as dark as 35% in front.
I have the factory privacy package (40% tint on rears) and I'm getting my front two tinted tomorrow to 40% also, to match.
The tint people here say that tinting the front 2 as well will darken up the car as a whole and therefore not require the rears to go darker.
I have a light tan interior so my interior will still show through, but if you have a black interior, the 40% all around will be nice and tinted.

As for the tint which blocks heat more, I'd look at it IF I drive the X5 in summer.
I will not, so I went with normal tint film. BUT tint guys swear it does make a difference in the summers.

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I swear, my cars are like a girlfriend.
Sometimes its a rough ride, sometimes its smooth motorin'.
Sometimes she doesnt like how i treat her and sometimes i dont like how she behaves.
BUT at the end of the day, she loves it when I am inside her.

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'91 850i
'05 X5 4.4i
'09 Clubman S