Both are a little odd. If I had to choose one I'd choose the first, I think. This is assuming you want to make an ordinary statement. If you want to be a little more poetic (!) I'd go for the second. The second would work well if you add on to the sentence (...to be what?) But as they stand I'd have to chose the first as the second begs the question, "to be what?" Why not change the sentences? There will be humans around for a long time? Humans will long continue to exist? Humans will (continue to) exist for a long time?

I disagree with the comment below and much prefer the second in fact... but it would need something on the end ideally... HUMAN BEINGS WILL LONG CONTINUE TO BE THE PLANET'S DOMINANT SPECIES!

To me, #2 sounds like you are trying to be deep and philosophical, or maybe poetic. But it doesn't sound great, especially not for everyday usage. I'd stick with #1, if that makes sense :)