I have both the 10 vol. and the 2 vol. Selected Shorter Writings sets, and For what it's worth I prefer the 2 vol. set. It consists of sermons, letters, book reviews, rants, misc. essays and articles grouped by theme. These volumes contain some outstanding little essays, and they give the reader a more personal glimpse of Warfield, the man. They also tend to have a more pastoral bent in their thrust and application overall.
It may be that the demands of seminary study have kept me from reading more of the 10 vol. set, but I have read some, and I think that I will still prefer the SSW in certain ways. One of the things that I have wondered about is how dated some of Warfield's scholarship might now be in vol. 6 on "The Westminster Assembly and its Work"? It seems like there has been a fair bit of scholarship done in that area since his time. Just a thought.
Inwoo,
The two volumes on perfectionism are actually vols. 7 & 8 of the larger 10 volume set. They probably have a set in the WSC library that you could check out.