Does your T520 have the i5-2520M?
Yes, it has the i5-2520M CPU ..
For very basic tasks, the improvement would be pretty small, although for heavy-duty stuff the performance increase would be significant. Have you considered upgrading the HDD to an SSD.
Right now, I have a 750G/7200 drive (I like the space). The T420s allows one to put an mSATA memory disk in the system as the OS drive. Can both the T520 and W520 do the same? I did that with my T420s and really like that.
Any other advantages to the W520 over the T520? Does the 16G make a difference? I do some heavy-duty stuff on occasion but usually I am just doing a lot of things all at once more so than heavy-duty processing.