2007 X5 vs Merc GL

[cpd1]

All,

Recently joined this great forum, I have a X5 3.0d on order for late Jul delivery here in the UK. It is going to be the main family hauler and my wife's daily driver. She currently has an X3 but with 2 kids and 2 car seats that doesn't leave much room, plus I have never been a fan and ride/quality are both scary...
Anyway, love the X5 look, have had few BMWs before. Form all accounts it is a great drive but wonder how it will translate on the low quality UK roads. I cannot drive it here until late April but I am also considering a GL 320 CDi, big car for the roads here in the UK so it might be fairly impractical. On the other hand, it is actually an inch narrower than the X5 and only 6 inches longer and it has 2 real seats in the 3rd row.
As I said I have a 4 and 2 yr old but with the school runs, every single time we try to have a 3rd kid it is really a pain and that would be the main occasional use of the 3rd row seats. Also when we have family visiting it is obviously a plus.
Performance is better on the X5 but only 1 sec on 0-60 so not a huge deal to me and certainly not for the wife...
The ride of the GL is great, off road creds fantastic but useless to me. (drove the car at Brooklands the new Merc showcase dealership and they have an offroad testing ground along with a full track nice!)
Never owned a Merc before and the interior is OK but nowhere near as nice as the X5 is (at least from what I can tell of X5 pictures). The space factor is nonetheless the key plus along with a great ride quality.
Clealry I am going to get BMW biaised views here but nevertheless would love to hear the board thoughts on the 2 cars...
Much appreciated
Thanks

C