Which NIKON lens do you prefer? The 18-70mm or the 55-200mm?

Which of those lens can produce a sharper image?
What other lens do you have in mind that produce a sharper image with a moderate range and not too expensive…
Do you consider SIGMA lenses as an option?

Not being a Nikon user I can't speak to these specific lenses but let me say this. The 18-70 lacks scope - in other words not enough reach. The 55-200 is too tight at the bottom so in order to get
wide images you have to changes lenses. To get reach, guess what.

I would recommend an 18-200 or a 28-200. Both are fairly wide at the bottom but offer reach.So, no changing lenses every time you want close or far. This will keep dust out of your camera and off of your sensor.

I use a Canon 1000D and a 28-200 - it serves my needs very well.

It depends on what you want to photograph! Jim's suggestion of an 18-200mm will give you a greater range, but the image quality will not be as good as a fast prime. I have the 18-200mm & it's useful for when I'm not sure how much room I'm going to have at a shoot, or if I'm not sure what I'll be shooting.

For more specific work, I use specific lenses - 50mm f1.8 or 85mm f.18 primes, or a 24-70mm f2.8 zoom.

For landscapes I tend to use a Sigma 10-20mm & for wildlife/sports (don't do this much, not really my 'thing') a Sigma150-500mm.

Sigma lenses are good - the 10-20mm is one of my sharpest lenses.

My photostream link is below - I always tag images with lens used, so feel free to have a look.

NB I don't have/use either of the lenses you mention as the 18-200mm covers the range for me & will be of similar quality to those.

Well if I wanted to take something at 200mm the 55-200mm is the clear winner, if I wanted to take something at 18mm, the 18-70mm is the clear winner.

Why are you comparing lenses that only overlap by 15mm? That's just bizarre!