I was debating this for the longest time.... I kept going back into
best buy and comparing the two. The main thing I noticed is that the 15" display feels
much more immersive when you use it for a bit and then switch to the 13". The thing that keeps putting me off from the 13" is how useless its native scaled 1280x800 seems. 1440x900 is the bare minimum I feel comfortable with, but I found myself using 1680x1050, though UI elements are quite small. When you jump back to the 15" the 1440x900 native resolution feels great, but the max scaled 1980x1200 is wonderful as well and I can see that setting being very useful for working with lots of sliders and such in Lightroom, Photoshop, Logic, etc
I actually found myself preferring the footprint of the 15". It just stays put on my lap better, and honestly doesn't really feel any heavier because the weight is more dispersed. It is definitely way more lap-friendly than the 15" cMBP.
I'll be using my machine for photography, music production, programming, and school work (writing and CAD). While I love how portable the 13" is, it does feel slightly toyish after adjusting to the 15", especially at its super cramped native-resolution. I just can't see myself editing photos on the 13" for long periods of time, even though I use to do that on my old 13" cMBP.
It's really funny, I was so excited for the 13" to come out because I thought it would be the perfect laptop. For a lot of people it would be perfect, but I think anyone wanting to do serious photography work should consider the 15" because those 2.1" inches makes a big difference. I'll carry my 15" with me everywhere just like I did with my old 13" than weighed slightly more.
You can definitely use the 13" for photography and design work, it's just going to be a much more intimate experience haha. The only way I'll consider the Haswell 13" is if apple manages to include a quad-core CPU and the option for 16gb of RAM.