WW(you)D? MacBook pro 15" or 17"?

[edgarbc1]

If I go 17 then I have to get new bag
But I might not get external monitor if 17 proves
To be big enough for my needs

If I go 15 a slight mod to my existing bag
And a likely candidate for external monitor
I would get high resolution screen option

17 is big. 15 is more portable

Thx for yr opinions!