nomis5 - 9:24 pm on Jun 13, 2012 (gmt 0)
When I designed my site for mobiles, the same question cam up - vertical or horizontal.
From a personal perspective I always use my mobile in horizontal mode for the internet but then discovered that the rest of the world uses it in vertical mode. I have no idea why because horizontal provides a far better experience, but who am I to argue with the vertical world!
Why not try 320 though instead of 300? And don't forget this, designing your site for 320 wide vertical browsers is designing it for almost certain obsolesence. Next year the ideal width will be 450 or whatever.