I have one of each currently. They are very different, but as per another recent thread here, the X3 is a lot more nimble (and frugal) compared to the X5. I would buy another either of the E83 model or whatever is coming next as a 2010 model year.

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Fastbuck
2009 F01 7 Series 3.0D SE Saphire Black/ Oyster
2008 E83 X3 2.0D SE Montego Blue/ Black
2007 E70 X5 3.0D SE Space Grey/ Beige - SOLD
2006 Land Rover Freelander Sport TD4 Zambezi Silver/Black - SOLD
2005 W203 C200K Sports Coupe Jasper Blue/Artico Black - SOLD
2004 W211 E320 CDI Avantgarde Brilliant Silver/Alpaca Grey - SOLD
2001 E39 530i SE Crystal Green/ Beige - SOLD