We've played a lot of moderns/near future with GHQ stuff. It's the most complete line of modern gear available with far more options than 15mm or 20mm.
We play primarily on a 4" grid. This makes life so much easier than trying to use rulers, especially for urban battles.
Unfortunately people seem to prefer larger figures so most of our recent gaming has been in 15mm. But I've been tempted to haul out the 6mm stuff again, especially our near future and sci-fi gear.
For rules we use something we built ourselves. Sort of a hybrid of the old Space Marine rules and Flames of War. Very simple and fast. Again, the square grid makes the scale even more enjoyable imo. 6mm troops can be sort of tough to move around with any level of precision. The smallest bump in the terrain makes them slide or topple. With a grid you can position them within the grid realistically relative to the terrain instead worrying about precise positioning for measurement purposes.