The 5.7x28mm cartridge is definitely a niche market right now, mainly because there are so few guns chambered in it (read FN five-seven and PS90). But to have a necked down rifle cartridge fired from an actual pistol (sorry AR guys!) - is quite appealing. The ballistics of this round are usually around 2300 fps with a 28 grain bullet and somewhere around 350 lb/ft of muzzle velocity - not the most impressive, but certainly has a use-case (ie. puncturing a tire for instance) and impressively low recoil which leads to better accuracy and faster follow-up shots.
Back to the question - the FN was first and it is a quality firearm, but the Ruger (IMHO) looks better and is in somewhere around $350 cheaper. That is enough to talk about if you ask me. Ruger is not a second-hand manufacturer either, so it seems like if you are in the market for a pistol chambered in 5.7x28mm, you are probably having trouble making up your mind (and rightfully so).
If you want to compare specs, you will be splitting hairs.