OEM for me if your sticking with the stock setup. The point in grooved discs is to allow gas/vapour between the pad and the disc to escape, can you really push the stock setup to those extreme's? I dont think so, way i see it less grooves, more surface area to brake with. I dont even think most proper performance brake setup's use grooves do Willwoods and such come with grooved disks? When I had grooved they made a horrible noise when braking, cracked from where the grooves started and i dont think they look any good when rust gets in all the grooves. I find the OEM replacements a much better balance between daily driving and spirited driving. I dont think the stock setup will ever be able to deal with hard driving, they fade very quickly, slam on from a ton and thats it, ive found this with both grooved and stock disks there just so small and thin. I do think though that the stock disks come back when pushed better than the grooved ones I had. The yellows and grooved I had always felt a bit glazed I never had good pedal feel with them, like they just didnt feel as if they were biting, stock set I think has a much better pedal feel. Ive even ditched performance pads and gone for ebc ultimax which are just stock replacements. Spot on from cold, no problems with sprited driving, die like the rest when hammered.