You really need both to match but I would say the lowers because they hold the weight of the vehicle by the springs and hold all the forward thrust when accelerating, and breaking. The uppers controll axel windup and side to side motion.
You can get adjustable uppers to adjust pinion angle, which will greatly improve launching. Start with lowers though if your just doing one set at a time.
Best in a matched set, as said; otherwise, lowers are far more important, also as said. I'm with them. On a sidenote- it doesn't pay to bargain shop for these unless you are very easily satisfied.
Maximum Motorsports or Griggs. I recommend them pretty much exclusively, and run only MM stuff on my own car; I'm sure others will chip in with a selection for you. Depending on how serious you are, you want something that won't bind. The only way to eliminate this is with hiem joints or spherical bearings, but use of arms with those will require you to run a panhard or watts link. They are also the most expensive, and transfer some noise into the cabin. I have them, and I kinda like it.
The next step down would be a three peice urethane bushing with a hard inner to prevent deflection and a soft outer to allow angular motion to combat bind. Not as good, not as expensive. There is a middlie ground between the two: the three peice bushing on the chassis end and spherical bearing on the axle end. Priced in the middle, and performance better than most people need. This peice is an excellent compromise, and highly recommended for someone who doesn't want or need to spring for the top of the line parts, and offer better performance than you will likely need on the street or even on the track while staying pretty quiet and smooth.
The bottom of the totem pole is solid urethane bushings, which will actually bind worse than stock rubber. MM has measure a wheel rate increase of 400% over stock with solid urethane bushed arms, and it only takes a 10% increase to upset the balance of a car. The only saving grace of these arms is that they are far stronger than the stock stamped steel parts, and the arms themselves do not deflect at all. Therefore, they are a big improvement up to a point. Once that point is reached, the fact that they bind even more than stock parts catches up with you. This is mainly a problem in the corners; if all you're concerned with is drag racing, they may be fine- but they will be more likely to damage your torque boxes than the other three options. Happy shopping
If you're interested in Maximum Motorsports, I'm going to be selling my MM standard lowers in the next couple of weeks. If you're interested, PM me or email me at fiveohfox@hotmail.com. MM also recommends the FRPP heavy duty uppers to compliment their lowers - I have those for sale too with brand new axle bushings.