As an old-school wargamer, I favor the the grid-based tactical combat rules found in the revised core rulebook more than the abstract rules found in the original core rulebook, although I must admit it does need some minor tweaking.
Of course, you can't run a chase/pursuit encounter with grids -- otherwise, you'll be running off edge of the map, even though freestanding, you need to shift playing piece so you can still move -- so I turn to the Spycraft chase system (core rulebook and Soldier/Wheelman Class Guide.
If you're not satisfied with anything out there, make your own.