Unless you're intending pushing the boundaries of RAM usage in your PC or are after bragging rights, then it doesn't really make any difference.
Given the choice, go for cheapest RAM that your motherboard can handle - watch the frequency though. Check your motherboard specs to see what freq. It can handle. There's not a great deal of performance difference between, say, 667MHz and 1066MHz, but it's worth staying at or above 800MHz if your PC can take it.