I have both but I personally did the SFCs first and added the strut tower brace after. Both made a HUGE difference in their own way but regardless of whether it's a drag car or road racer I think everyone will agree the SFCs are almost a requirement.
Subframes hands down. Your entire car twists and flexes in the middle and that's between the front and rear wheels where it'll make the most negative difference. Strut tower bars only help to keep the front end stiff on corners. You'll benefit from subframes in every driving situation but both are a good investment.
I have one car with a strut tower brace, one with nothing and one with STB and SFC's. The last one is amazingly more solid feeling than either of the others.
Gotta buck the trend here and say that chassis reinforcement is over-rated.
I have actually REMOVED parts of the subframe to reduce weight and increase power-to-weight ratio. It's more about removing stuff than adding stuff.
I went full-bore and also removed both front braking systems. Those calipers weigh a freaking TON!!!
I typically run only one quart of oil and only fill the radiator half full of coolant - these are necessary systems otherwise I wouldn't use any (more weight savings!!!).
If you want to go hardcore, remove all the glass (other than the windshield of course - that would be silly).