OP. I currently have a 13 and 15" with maxed out specs, both which were bought solely for music production.
One 13" mid 2014 Retina with 3.0ghz i7, 16gb RAM, 1TB PCIE. One 15" mid 2014 with 2.8ghz quad core, 16gb RAM, 1TB PCIE, with discrete and dedicated graphics, which matters SOME.
I use Logic Pro X with mostly plugins for installment libraries and AUs for mixing, mastering, and effects. Sometime I use native effects and libraries. With that in mind, I guess I will take on the task to help you in the best way possible, by writing up my experiences with not only these two Macs, but my setup as well. I will start by listing the plugins I use, by brand.
You will need JBridge, or 32Lives for some of these. I recommend 32Lives.
Instruments:
FabFilter
LennarDigital Sylenth
Native Instruments Kontakt/Battery 4
Spectrasonics Omnisphere
U-he Zebra
Waves
Xfer Records Serum
XLN Audio Addictive Drums 2
Audio Effects:
Audio Ease
Cytomic
FabFilter
IK Multimedia
iZotope
Native Instruments
Nugen Audio
Ohm Force
Plugin Alliance
Softube
Sonnox
SoundToys
Valhalla
Waves
The part of my setup, that matters, consists of two to three monitors (external). The third is almost never used, but comes in handy sometimes. Usually I just have another Mac on that third monitor. The dual monitors are an extended display made possible by the dualhead2go from Matrox. There are advantages to having this setup, for me. If you plan to use your mac with a desktop setup, this matters. If not, it's irrelevant. Regardless, having two monitors, with or without the Matrox, could slow your Mac down. Barely at all, but you must know this. With a 13, it will be more noticeable.
I bought the 13" first because I needed decent power, with a lot of space (as I hate relying on portable hard drives), but it had to be portable, and powerful enough for a desktop setup. I found it to be fine, for a while. You will need to freeze tracks and keep in mind to run effects on bus as much as possible, depending on your amount of tracks and if you're using native or plugins. For instance, I made one composition with mostly LP instruments and it was more than fine. However it started to tax the machine even with on board EQ. Then, I have a simple composition that is still in it's infancy, but uses several plugins, if I recall, is about 6 tracks, that will run the CPU into the ground.
In relation, all I need to say for the 15", is that for that same 6 track composition, it's like it's nothing. I ran one of my most intensive and nearly finished pieces on both macs with exact same setting all the way down to the wallpaper, and found that when the 13" CPU was maxed out, the 15 still had more than 60% room to create, and about 80% on the 4 cores created by hyper threading. I can skype, run an intensive composition, and surf the web for tutorials (Sometimes I still need help with some plugins) on the 15". On the 13, I made sure to use the other Mac for skype or internet because there was a lot of conflict if I used it alone.
What does this mean for you? Almost nothing. Factually, you will be safer and more productive on a 15". Otherwise, it all depends on portability verses what you're doing, musically. My most intensive tasks are writing scores, and my least are simply editing songs from clients to help write lyrics. Most of the time, the 13 was fine even while tracking vocals. The thing that got me, was when I was limited, it was a big deal and it stopped me in my tracks.
If you get a 13", I wouldn't recommend against it, but KNOW your plugin's CPU requirements and take all necessary steps to maximize your Mac's capabilities. Bus, freezing tracks, closing all apps that are not absolutely required for music production, buffer size... there are many things that can be done to help you on a 13". I made due with a 2.4ghz 16RAM 2010 for a while, but I had to be incredibly alert as to my process (or some would say "work flow").
Last, I would get the best 13" possible. Some would say that .2ghz won't matter, but it will and you will need all the help you can get.
I hope this helps, and happy composing.