Depends on the series and strike characteristics of the key date. There are series where I'm not too picky about the XF or AU distinction in the keys and will go for the best coin at a reasonable price. Buff nickels are one right now- the 26 S has a huge jump in price from Xf to AU and I'm going for an upper end xf. In the Standing Libs with the horrible strike characteristics of the 27s I would go for a quality XF because the wear would help distract from some of the strike blemishes that all of those coins have (weak shield for instance).
In Lincoln WC I went exclusively for AU since I like strong details on the face and ear, and little wear on the wheat stalks.
It's actuallly the semi-keys that have stymied me on the buffs and Lincolns. For instance a well-struck, reasonably priced 26 S Lincoln took over a year to find and I'm at 1.25 years and counting searching for a decently struck 27 S Buff that is not priced at an outrageous premium. For the latter either my standards or my wallet need to change ...perhaps......