Absolutely not true. For mainstream usage and single threaded applications, there will be virtually no appreciable difference. Of course if you are running more intensive programs that utilize the superior quad-core processors then the difference will be much greater. But to imply that the 13" feels dated is simply not true at all, especially since both machines share the same PCI SSD.
In regards to the original question here are my thoughts. I think there are two ways to think of portability as it relates to current notebooks. Footprint and weight. I originally had a 15" Haswell rMBP and later opted for the 13" model so I'm familiar with both.
The weight for both is negligible and for all the talk about the Air being so light, unless you have a physical limitation of some sort, both rMBP models are fine to carry around daily. The footprint (overall size) of the 13" is where it shines. While the 15" is much thinner than its cMBP predecessors, it still has the length of a bigger machine. The 13" has a more compact overall size than even the Air and there is very little wasted space (i.e. the keyboard on an island of the 15"). The 13" will easily fit in almost any type of pack and it is very comfortable to simply carry around with one arm as well.
I think the minimal footprint of the 13" rMBP is actually its greatest asset and I actually prefer the additional 1/2lb of weight than the Air because the block shape (especially the thicker screen) feels much more durable and solidly built to me. The 15" is an incredible piece of machinery with its additional screen size (which seems much bigger in real usage from the 13" than just 2.1") along with all that additional power under the hood. But assuming Iris has enough juice to meet your needs, I think the 13" is the much better buy for this generation if portability is one of your primary factors.
