The BA3520 is 5 more than the BA3515. 

Both motors are very powerful.  It might come down to which timer system you are using and the weight of the model. 
I have heard some scuttlebutt of folks using the BA3520 with Igor's newest timer variant and a particular version of Igor's props. 

The BA3520 is ~8oz and the BA3515 is ~6.5oz.  A 1.5oz weight change at the end of the nose might be compelling enough to select one over the other as needed.

Brent, Igor
Thanks for the reply /info.
I assume battery usage might be up too. ... but for a given plane it takes the same amount of energy to fly with one or the other .... just the internal losses being different...... straight and level flight, same prop, anyway.  Maneuvering flight, ....  maybe, the 3520, if the 3515 is stressed.
Frank

Unlike R/C where you can vary power by using a throttle we tend to operate in a limited power band.  For a model of a given size, given line length and desired lap time the power needed to achieve level flight will fall in a relatively narrow band with some variation due the prop used and how efficient the set-up is.  Since your top speed is constrained (i.e. lap time) the only way to increase power is to either increase the diameter of the prop, add a blade, or do both. I normally do not think about pitch since I generally run either 4" or 5" pitch (generally XOAR wood props), once you have your line length and lap time sorted out the pitch sorts itself out.  In our case picking a motor that has a higher peak power rating may not really help you if you do not need that much power.

The other thing to keep in mind is your current consumption.  Our flights normally last about 5.5 to 6 minutes so our discharge rate is limited to about 8C or so on average so that you do not over discharge a pack. A 10C discharge rate will "empty" a pack in 6 minutes and hence why we need to be below 10C. This affects how large the battery needs to be in terms of mah and also what voltage you should use. In general, using a higher voltage pack will allow you to use lower capacity packs.  As an example, on the .40 size Legacy that I have, the 6S 2200 mah set-up is lighter than the 4S 2600 mah set-up it replaced.  The current levels were lower so I could use a 35 amp controller rather than a 50 amp ESC and I was also able to use a smaller and lighter motor (with a lower Kv).  The difference was not huge but meaningful, about 1.5 -2.0 oz. (I also fly R/C pattern and there was a discussion some time ago  about how much one should be willing to pay to save 1oz of weight, and this was on a 11lb model....).

I am just an average pilot and the Legacy needs about 110-120 watts/lb to fly the sequence.  Level flight requires around 60-80 w/lb with higher values of about 150 w/lb when maneuvering. If you estimate about 150 w/lb at peak, a 64 oz model would require a motor capable of about 600 watts.  Give your self some room and call it 800 watts.  In this context, the BA3515 should be more than good enough.

The Kv you pick will be a function of pack voltage and what size prop you would like to spin.  On a 60" wing span model I would probably run a 13" prop and pick a pitch to give you the lap time you are looking for.   If you have a particular model in mind, knowing what it is would be helpful.

Good luck.

PD.  I have played around with Sunny Sky motors and BadAss motors and my impression is that the Sunny Sky motors are more efficient but the difference is not huge; maybe 5% at most.