OK a little history. The 5.7 was developed by FN as a candidate for the US Army's PDW (Personal Defense Weapon program). A PDW was meant as a weapon for troops NOT needing a rifle; but better served than just having a pistol--picture mechanics in a combat area working on vehicles with their rifles leaning against a tree or locked in a rack. The PDW was meant to be something they could have with them, that had a 200+ meter range with the ability to defeat (soft) body armor.

HK produced the a 4.6mm cartridge in a weapon similar in size to a M10 SMG. The FN design is the P90. While the P90 is more of a shoulder fired weapon, the HK (like the Mac 10) is a "pistol" with a folding stock that can be holster carried.

FN seeing something of a civilian market also produced a semi auto version (PS90) and the FN57 pistol. As AP handgun ammo is banned in the US, the neutered civilian ammo fails one of the primary intended pluses of the cartridge especially with the proliferation of cheap body armor and its use by criminals. The cost of the FN rifle and pistol, as well as the cost and limited availability of ammo had almost ended the cartridge.

The Ruger pistol saved the cartridge, or at least has breathed new life into it. Its a low (read that as no) recoil cartridge that moves at amazing speed. Plus it packs small into a magazine. A mag of twenty fits into a standard grip size handgun. The FN57 is the current handgun of the US Secret Service.

Comparing the FN to the Ruger, both guns are of similar size. Thus far Ruger has not released any magazine beyond the standard (20 rnds). The FN57 is available with 20 round mags and an extended 30 round mag. The FN pistol sells for $1500+/-; while the Ruger lists for $600ish. The Ruger is nice; but the FN is nicer. Is it worth double the price? IDK.

Ultimately they are both toys which do you prefer, a race car or racing stripes on your pick up?