Price? Generally speaking, I believe that the 135 will be $35,000 while the 335 is a about $45,000 to $47,000 though I have my hopes of scoring a brand new 2008 or 2009 335xi for $41,000 once my 330xi lease is up 9/08.
The 135 is tempting, I will hold out for the testdrive and the see how I like the fit. I'm also really curious if the weight trimmings on the 135 make a difference.
My question is, does the 135 even come with an all wheel drive option? I don't think it does. This may be what tips me towards the 335xi.