On my mtb the saddle width is 155 mm. Specialized shop measured me for that. I was told that on a road bike because of the position i should get a narrower saddle. Do you agree? Thanks.

Short answer: yes. With more analysis, however... 155mm in and of itself means little. Where the width is measured is of importance. It is not standardized.

Your back side will,be the same in both cases right?

For the agressive downhill rider that moves their Butt down and back behind the saddle
there were some made with the back corners not there .

NB: You can take saddles off the peg wall in the Bike shops , and actually Sit on Them.



Sitting Up higher , can have people preferring a wider saddle ..

A race bike If you put out the effort most of your weight is pushed onto the pedals.

thats why race bike saddles can be so much thinner and harder . (but Lighter)