what are the cons and pros of both ?

I got 245 35 18 , the guy at the shop told me , that I can put 255 35 18 ,

it will be 0.5 inch wider ,,, is it a good idea? is it enough?

or shall I get 19 '' ?

Impossible to answer your question w/out knowing the make/model/year of your car...

last time i changed tyres i moved from 245's to 255's this is no hassle change and for me the tyres actually worked out cheaper,but i haven't noticed much difference grip wise and the appearance isn't much different either ,so i'm thinking of going with the 19" wheels and 265 tyres

its an SLK 55 AMG

Thank you for the review ... I dont know what do , I guess i'll do the same as you ...

iam thinking of the SL 65 rims 19'' ... does it affect the transmission ratio?

I dont care about comfort as much as handling , road holding and steering feedback...

Though not exactly the same situation, I went from 225/60QR-16 OEMs all around to 245/45WR-18 Goodyear Eagle F1 GS-D3 XLs front and 275/40YR-18 Goodyear Eagle F1 GS-D3s back, and had no problems at all. I think I might have had fitment problems w/ 19" wheels and tires, but your ride is smaller so I am not sure exactly how to advise you. I am sure there some folks here that have exactly the info. you need. In fact, you might consider posting this in the SLK55 AMG forum for more car-specific advice.

THANK YOU dude

You're welcome. Us "Seniors" have to stick together...