I sold my 2008 2.4 MBP in December after buying a 2.3 13" MBP last summer.
Yes, it's faster and smaller, which I like a lot. The boot-up time in Lion is much faster than Snow Leopard on the 15" MBP. The 15" is too big for how I like to travel, which is LIGHT.
4GB RAM may be plenty depending on what you do; try it first before upgrading.
I suggest using the excellent Power Support anti-glare film for $35. Apply it as soon as the MBP comes out of the box and skip having to clean the display before installing the film. When I compared the anti-glare film with my anti-glare 15" MBP I couldn't see a difference... Reflections are gone so No Worries about the ambient light.
I use the 13" MBP with a high-quality 24" NEC display. I think that I am done buying desktop Macs.