My advice as well. However, since a lot of these cards are relatively cheap, I usually try to get all manufacturers. To make it more interesting, you can throw in the oddball cards in the mix since the fleer/donruss/score/ud/topps cards are fairly easy to obtain. I'm a greg maddux collector...so, i got all of his rookie cards produced by the standard brands, but also went after his oddball cards, which took more time to research and obtain (since they dont go up for auction as frequently).
But, if you simply just want one RC card for each player..go with the one you find most appealing.