I agree with Tek, get the faster lens every time (f2.8 or better) they focus better in low light, usually have better optic and therefore tend to be much sharper – plus they just look sexy.
I'm a Nikon mount person (Fuji and Nikon bodies) and currently own a 17-35 f2.8, a 28-70 f2.8, the new 17-55 f2.8, a much loved 70-200 f2.8 with vibration reduction, and for reaching out and touching someone, the legendary 300mm f2.8 – there will never be a zoom lens (that you can carry) that will compete with the sharpness and speed of this monster.
I carry the 28-70 and the 70-200 with me all the time - the older 28-70 is much sharper and focuses faster than the newer 17-55 - an added plus is the weight helps keep camera shake to a minimum. I like the 17-35 for tight spaces and some exocentric glamour angles. For porn, it’s almost always the 28-70 – fast, sharp, focuses close enough to count the ingrown hairs on a moles ass, and is wide enough to make even the most diminutive penis’s look larger. And if you’re shooting porn, what do you need a 300mm for? If you can’t smell it, it ain’t no fun!
If you do any serious enlargement (larger than 8x10) – always use a tripod! Especially with lenses longer than 100mm. When I used to teach, I could always spot the student that had tried to shoot a sunset with a 200mm lens and no tripod – even on 4x6 inch prints.
An additional advantage of two bodies is that digital slr’s are VERY susceptible to dust – change lenses no more often that absolutely necessary and NEVER purchase a store demo digital SLR.
Anyway, my 2cents