Only you can make that call -- but if there's a restocking fee involved returning the other RAM, the answer is definitely no. The manual for your system mentions support for 1066 and 1333 MHz  -- not 1600.  And as above, the difference between the two is so slight it's really not worth the effort or even the $5 cost. You will never notice a difference in the real world use of the system.
XPS 15 L502xi7-2860QM (2nd-generation Core, Sandy Bridge) Hello: Buying new RAM, I'm wondering whether it's worthwhile to choose speed 1600MHz vs. 1333MHz. 16 GB works in this laptop, I've tested it. Intel specs, Crucial and others say that this system supports speed 1600MHz. There are posts in this Dell forum from owners of this laptop who say their 1600MHz RAM indeed runs at 1600MHz. Is getting 1600MHz (PC3/PC3L-12800S, CL11) worthwhile versus getting 1333MHz (PC3-10600S, CL9)? The price difference is less than $5.Any reason not to choose 1600MHz?
XPS 15 L502x, i7-2860QM. 16 GB RAM, is 1600MHz worthwhile vs. 1333MHz?