If it has to be a 4 cylinder, you want the M111.

Much better in every way, and you don't have to mess around with mechanical CIS-E injection misery

 

I have heard that m102 is smoother and quieter than m111.

 

They're both 4 cylinder motors without modern tech like balance shafts, electronically controlled engine mounts, etc.

4 cylinders from the W124 era are inherently lacking smoothness.

If it is smoothness you're after, you need to step up to at least a 6 cylinder. Also, the M102 has to work hard to move the mass of the W124 and revs high on the highway.

FWIW, think of the M111 as a upgraded M102. The M111 bases its roots on the M102 in a similar way as does the M104 to the M103.

 

isn't the M102 more or less a 4 cyl version of the M103? and the M111 is roughly equivalent to a 4 cyl M104 w/ dohc, HFM, etc ?

 

^Yes, pretty much.