Personally, I went with the mbp because of other factors. If a 13" mbp had been around, I probably would have gone with that and used the difference in cost to buy an external screen. Think about from a logical perspective; laptops are meant for travel. A smaller screen has virtues that are very important for people who travel a lot. You also can't typically do a lot of work when you're moving around a lot. Once you do get to your home/office to do some real work, a 23" screen gives you more than enough space to work with.
All that being said, the mbp has more work area, but the same pixel density. What does this mean? It means that your text size and such will be the same. What changes is how many windows you can see at once. Like I wrote above, how many windows do you need open when you are traveling? This is one of those cases where the grass is greener on the other side. Trust me, your 13.3" macbook screen is actually superior in some ways to the 15.4" mbp screen (except for the glossy part-can't stand glossy).
All that being said, the mbp has more work area, but the same pixel density. What does this mean? It means that your text size and such will be the same. What changes is how many windows you can see at once. Like I wrote above, how many windows do you need open when you are traveling? This is one of those cases where the grass is greener on the other side. Trust me, your 13.3" macbook screen is actually superior in some ways to the 15.4" mbp screen (except for the glossy part-can't stand glossy).