Here's my take on the issue:
I've owned 3, 17" laptops in the past few years and done extensive work, play and travel with them. That being said, I'm opting for the 15.4" mbp this time around for a few different reasons.
1) Size and portability. Most people want 17" notebooks because they are looking for a DTR (desktop replacement and need a display that is adequate to perform most of their work/leisure activities. This comes at the tradeoff of size,bulk and weight and decreased portability. This was my fundamental philosophy during the past few years, but my argument now is this... Why not get a portable 15.4 notebook and simply use an external display for when you are at home? Even when I was using my 17" screens I found myself wishing I had a proper "large" display which is why I'm using a 24" Dell LCD at the moment and intend on using it with my MBP. Lugging around a 17" notebook is a pain. It's unwieldy on planes, doesn't fit on desks very well, can't be set up very easily on coffee shop tables and well.. you get the picture. I found myself leaving my laptop at home more times than not and using it strictly as a stationary DTR which begs the question... Why did I get a laptop in the first place?
2) Screen resolution. This is purely relative because I think all of us have different thresholds of tolerance as well as different levels of vision.

The majority of my 17" screens have been 1920x1200 and that has always been a difficult resolution for me to work with on such a small screen. It simply hurts my eyes after a very long computing session and text/icons end up being uncomfortably small for me. Sure, it's more screen real estate and I can open more documents side by side, but on a 17" screen it's almost too much. 1920x1200 on my 24" widescreen is just perfect. 1440x900 on a 15.4" screen is also perfect for my particular tastes. Also, if you're a big gamer (as I am), this plays an important factor. Generally speaking, you want to play games at native resolution on your LCD's so that you don't sacrifice image quality with pixel mapping at non-native resolutions (yuck). 1920x1200 is fine if you are playing on a more powerful notebook card (7950gtx, 8800m, etc..) but 8600m GT isn't going to cut it for anything other than older games at that resolution.
3)Form Factor. I haven't found a 17" notebook yet where I was happy with the keyboard placement and found it comfortable. Most of the time, the 17" depth is causing you to lay a large portion of your hands/forearms over the chassis to reach the keyboard. You end up with this massive lower chassis underneath the LCD comprised of nothing more than a keyboard and trackpad. That's a huge amount of space and I don't care how well you place everything, you end up with tons of redundant useless space. I don't like this and again, personal preference.
So, my argument (with myself) for buying the 15.4" mbp is that it's a more than adequate resolution for my needs, it will provide me a much longer lifespan with games at 1440x900 (vs 1920x1200) and if I need a bigger display, I can always hook it up to my external. (The price difference between 17" and 15" allows you to easily get a 20" widescreen LCD) It retains portability with it's size/weight and allows me to carry it around more often.
So, to make a long winded opinion very brief: I'd suggest buying a 15.4" and getting an external LCD later on if you need additional screen real estate. Better yet, go to an Apple store and play with both computers. That's what won me over to the 15.4" because it had been so long since I'd used a smaller screen that I automatically assumed that anything smaller than 17" wasn't big enough. When I started messing with the 15.4" mbp in the store, I realized that it was just about perfect.