This is more of an hardware question, but since pretty much the only hardware intense thing I am going to do with my computer is going to be photoshop, it is also, in a way, software.
The question is: in your experience, how much does an SSD improve performance over a traditional hdd in comparison with the improvement of a better cpu?

The context is that at present I have a 2010 15" macbook pro, which struggles a lot when my city maps gets big. I am in for a replacement in the next few months, and from a pure work-related perspective (mostly academic writing) a 13" macbook air would be probably better than a 15" retina mb pro. It would cost less and have better portability and battery life. Of course the bigger and higher resolution display of the MBpro would be nice, but it would not in itself justify the higher $$$
However, I am wondering, would the 2013 air, thanks to its SSD, be an improvement over my current laptop, for working on large photoshop files, even though if the cpu is comparable if not slower?

Having both a small laptop and a desktop is not an option since I am often travelling back and forth for long periods. Also not-a-mac is not an option because of preferences and softwares I actually use.