Not sure why you would need to choose between the two. They're both good. And then why leave out Letterhead Fonts?
SignDNA tends to feature sign artists with national reputations. SignFonts is the work of one person. For what it's worth, Steve C is that person and is an active and helpful member here. Letterhead Fonts has also contributed some useful tutorial material here. Dave Simpson of SignDNA is not a member here.
So if you feel you have to choose, I would choose SignFonts and also have a look through the library at Letterhead Fonts. Links to both sites are at the bottom of every page here at Signs 101.
SteveC's Signfonts immediately became my first choice to use after installing. They make possible signs with what the late Mike Stevens in his indispensable book on layout called "eye appeal". Worth their weight in gold.
Each has its bennifits. The fonts you pick depend on the types of layouts you do. A good variety of fonts for all general use is recomended. You can buy just the font you want or need. Just my thought.