Yes, I meant the Dell U2515H. Sorry about the confusion.
Oh. Then the answer is that there is a LOT of difference between those two.
U2415: 16:10 aspect ratio WUXGA 1920 x 1200 resolution
U2515H: 16:9 aspect ratio WQHD 2560x1440 resolution
So unless someone for some reason wants or needs 1920x1200 as their max resolution, the 2515H is a better solution for higher resolution performance.
You can find the specific specs on these two pages:
http://accessories.dell.com/sna/productdetail.aspx?c=US&l=en&s=dhs&cs=19&sku=480-ACRZ
http://accessories.us.dell.com/sna/productdetail.aspx?c=us&cs=19&l=en&sku=860-BBDY