You can be both. But I think there will always be a place for speed in any sport. strong guys are a dime a dozen....but a quick guy? one of the most difficult players to take on is the guy who can out run you.

why not both?

I think in my opinion being quick is better. Even if you're strong, all you need is a guy who knows the technicalities of tackling to bring you down fairly easily. I play fullback and there were many times where I had to tackle a strong forward who could easily crush me, and times where I had to deal with an elusive winger or a great stepping fly half who could trick me, and judging from experience, it was harder and more challenging to hit the quick guy.

BOTH

Depending on the situation both can help