it depends how long she will keep it. If its a 3 year lease go with the 750. if its a long term purchase get the S550. The reason why is the benz is a new body and will be current for the next 7 years or so. the 7 series will be redesigned in 2 or years. so if she is going to have it for more than 3 years go with the benz it will look newer longer.

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Dinan M3

2006 X5 4.4i Sport Current Vehicle
2006 CLS 55 AMG Current Vehicle
2010 FORD F-450 King Ranch Current Vehicle
2012 GL 550 Current Vehicle

1999 M3 Dinan S3 SOLD
2001 M3 Dinan S2 SOLD
2002 M5 SOLD
2004 X5 4.4i sport SOLD
2007 335 XI Lease ended
2009 E92 M3 SOLD