I bought a 4115 this week over a 2520. For me the reasons were availability and price. My local dealer only had a "demo" 2520 on loan from the regional sales rep. They sold more than one during their Open House last weekend because of this demo model. I was "sold" on the 2520, too. Until they told me that I'd have to wait until June to get it! My grass and driveway can't wait that long. So I bought a very slightly used 4115 (71 hrs) that they had just received in on trade. The differences between the 4115 and the 2520 are minor (IMHO). And I got the 4115 for $2K less.
It's a strange time for JD pricing on these tractors!
The MSRP for a "new" 4115 is $2,500 HIGHER than the MSRP for a "newer" 2520. (How'd dey do dat?)
Only because mine was a "used" tractor was I able to beat the price on the 2520. But, because my 4115 will be delivered in a few days, instead of a few months, it was an easy choice for me.