an inline6 is perfectly balanced on all forces and moments (twisting forces). This characteristic is shared with the flat-6 and the 60-degree V12 (although this can be achieved with a V8 via counterbalancers)

the 60-degree V6 and 90-degree V6 need counterbalancers to smooth out the vibrations, moreso in the 90-degree one.

the inline-6 has an even firing order, i.e., the exhaust pulses are regularly spaced. The 60-degree and 90-degree V-sixes will have pairs of exhaust pulses bunched nearer each other, and will therefore have a rougher sound relative to the inline-6.

the V6 has a shorter crankshaft, which makes it easier to make it robust than an inline6. Witness the F1 turbo V6 engines a long time ago. BMW's first M3 engine was not an inline6, but an inline4, because it was more robust than its current inline6 then at high rpm.

an inline-6 has a longer block. When subjected to a change in temperature, the length of an inline-6 will increase more than a V6 - this is probably the reason behind the head gasket problems of the M103 and M104 (i.e., the block and the gasket expand at different rates, but this is more pronounced on an inline6)

there are also packaging differences:
* an inline-6 has a hot side (exhaust side) and a cold side. The heat-sensitive components are placed on the cold side.

* the valley in the V of a V6 is a natural place to put a supercharger. The valley is bigger on a 90degree V6 than a 60degree V6.

MB's main motivation for going for a 90-degree V6 as against a 60-degree V6 (or an inline-6) is engine modularity. If they need a real answer to BMW, they will need to re-develop their inline6 - for me at least ...