Thanks for the input, by the way below a nice table relating display sizes resolution and dpi :
taken from :http://www.prismo.ch/comparisons/notebook.php
so 1920 on a 17" is 133.3 dpi and 1680 on 15.4 128.6 dpi , so a bit less ...
But I think I will go for 1680 , if all the OS and apps were scaling properly though I would like a 300dpi display, at least !
(after all good quality printing on paper for text goes up to 3200 dpi ! )