530i is a better choice because:
-less expensive
-faster than 525i (the slowest)
-i6
-save a little on gas
thats why bought mine few months ago. i chose 530i over 545i because of those reasons. I didn't even check out the 525i. sweet vehicle. check for open recall on any vehicle you intend to purchase. make sure you get sport package. make sure you go to e60.net forum for further information. (i am sorry for mentioning other forum, but since they exclusively for e60 i thought it will be ok, let me know if i have to edit it)

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2002 X5 3.0 Siena Red
Premium, Cold Weather, Rear Climate Packages, 19" Wheels
Aluminum Running Boards, Clear Tail Lights, Clear Side Markers
Titanium Kidney Grills, Titanium Trunk Lid, Xenon Headlights
4.6is X5 exhausts, 35% Tint, Aluminum Pedals
2005 530I Mystic Blue
Premium Package, Sport Package, Xenon Headlights
18" Sport Wheels, 35% Tint all around
Mods to come: M-Tech Front Bumper, M5 Side Skirts, M5 Rear Bumper
Quad Exhausts, Style 172 19" Wheels