I'll say from my very new experience but probably one month of research before acquiring the money to buy a fixed gear that there doesn't seem to be much difference in the frames of these bikes except for
lugged versus non,
steel versus aluminum (at least in this price range),
threadless versus threaded,
straight fork versus curved fork
Add-ons for commuting like braze-ons, brakes and the like versus pure track frame with zero holes,
Road Geometry Versus Track Geometry
Tire clearance Versus Bar Spin-ability.
Any one with more experience will probably say there are more differences then that but to me most of that seems pretty trivial when it comes down to it and probably nothing you don't already have unless you want a spiffy paint job and actual dropouts versus conversion drop outs.