Well, the M135i have gotten great reviews in Europe and UK, Chris harris said it was a bargain.
So it's really that good!
As for the M2, it should be great, as the 1M was and still imo is!
But it is a different platform, as the 1M was more "old" school, that doesn't mean the F2* is worse though, but different.
So choose your car for your needs, taste and wallet.
You can't go wrong with either imo.
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3 series: E30: 89 325i, E36: 92 325iC, 94 320iC, 96 320i Convertible, 94 320iaC, E46: 99 320ia, E90: 06 318i Le mans, E91: 07 318dt
5 series: E39: 97 520ia, 01 530Dat, E61: 09 520Dat
X series: E83 05 X3 2.0d, E84 10' X1 2.0 X-drive DA, F25 13' 2.0da X-drive.