What's better: long or fast?

I'm always somewhat shopping for things that interest me. One of them is camera lenses. I can't help it, I just like pretty pictures.

Nikon recently announced a 18-140mm F3.5-5.6. This would be almost our the range of our 2 kit lenses of 18-55 and 55-200. So, it's more of a "every day" lens without any bump in speed (max aperture). Tonight I'd discovered that there's a Sigma 17-70mm F2.8-4 lens. It covers the range of our "wide" zoom and then some, plus it's much, much faster.

So, hypothetically, if I was to get an upgrade lens, what would I go for: longer or faster? Both have advantages. What I really want is an inexpensive, light weight, small, 15-300 F2.2 lens. You know, something that probably defies the laws of physics as we know it.

Perhaps I just have to finally suck it up and start shooting in raw and spend some of my time doing some post processing. *sigh*

In the meantime, here is a picture of a flower.

I believe that this was just watered by a great dane.