tough one. i thought there was an article comparing the s2 4.6 vs the 4.8 posted not too long ago, both were pretty much dead on the same performance wise...there were significantly less 4.6s sold than there are 4.8s, so it really boils down to a) if you wanna save the 10k b) if you want the latest and baddest c)where the warranty mite stay the longest
good luck w/ your choice; i dont think you can go wrong either way...

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2008 E82 135is JB2 Alpine White / Lemon
2008 E90 335i JB2 Monaco Blue/Grey
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2003 E46 M3 SMG w/ VMR V715 19's Blue/Black