I used to work on a tablet pc with only 512 levels of pressure sensitivity and when I switched to the Intuos there was some difference. I especially liked I could customize the pressure beyond just soft hard, so yeah there is a bit of difference but I don't think you'll notice it much. The nicest thing is the buttons and the screen popup when you forget a shortcut.
As for the size, I was a bit worried Medium would be too small but I couldn't buy anything bigger because the medium could be set on top of my laptop keyboard perfectly. When I first started to use it I realized it was huge, even if the usable area seemed small, it seemed really big for a while. Occasionally it feels to small but then I turn on the precision drawing and it's great. I can make huge strokes for a smaller area of the screen.
I'm not quite sure how big the small one is, but if you don't mind the size of your current tablet and it's close to an Intuos 5 small and you want to save some money than I'd get the small.
I got mine a couple of months ago because my Intuos4 died. I like it better than the 4 - i find it more flexible. I also work on a 15 inch laptop but my first tablet was Graphire 3 and I definitely feel the difference in terms of size and pressure. The 5 allows me much finer details.
I have never worked on the I3, though, so I can't comment on that. However, I do highly recommend the I5 if you are doing artwork taht is rather painterly or requires lots of fine detail. Of course, if you are comfortable with the I3, that's perfect too.
well, the I4 was my first tablet with replaceable nibs but it eats them up to a certain point rather fast (again, I like firm pressure so that helps eath them up, too) . However, I haven't noticed this affecting me much to be honest.
Initially yes..it wears down at an alarming rate (don't panic). I am still on my first nib and have noticed that the surface isn't as textured as it was when I first got my my tablet. Either because the surface got much smoother over time or because I am applying less pressure.
You can definitely tell the difference between 1028 and 2056 pressure sensitivity. I feel like I am completely in control of everything I do.
I have an intous5 medium and personally like the larger drawing surface...however if you are used to a smaller sized area and want to save some money then by all means go that route. My recommendation is to go with the medium, you won't regret it.