While tradition dictates that the crest be used, I'm a fan of rings that bear the actual escucheon. Correct me if I'm wrong, but as I understand it the crest is "personal" and can be changed (even by the direct heir) in later matriculations, while the shield always remains the same. Since signet rings are both symbols of the family and generally expensive, they are often passed down from generation to generation; it just doesn't make sense to me to engrave a ring with something that may be changed down the line, however rare such a change may be or however traditional the engraving of a crest may be.

That, and I like the look of the shield on the ring!