imagemeister Loc: mid east Florida

If you have a use for the lens NOW - get it ! I would do a focus calibration on it at 200mm if you can or at least test it - to make sure you are getting the best IQ out of the lens - later, if not up to your expectations maybe get the Nikkor.
Buying , selling, testing lenses is all part of the "game" .....at least for me .
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imagemeister Loc: mid east Florida

CatMarley Loc: North Carolina

The increment in f-stops is not linear but logrithmic, so one f-stop (from 3.6 to 6.3_ is a big difference in the amount of light. As for the actual lenses you mentioned, the Nikon 18 - 200 is the choice. I had one and loved it until I got my Fujis and gave it away.

Najataagihe

https://www.ebay.com/itm/Nikon-AF-S-DX-Nikkor-55-200mm-F-4-5-6G-ED-VR-II-Lens/293195224997?hash=item4443cc17a5:g:-cEAAOSwiBBdXAys

Looking to save some cash?

Take a look at this lens.

It starts at 55mm, not 18mm, but it is a fantastic little lens for next to nothing ($65).

camerapapi Loc: Miami, Fl.

Let me start by saying that if you had to buy one of those three lenses I would recommend that you go with the lens that was made by the manufacturer for your camera. If money is a problem then you have to make your own decision.
There is no significant changes between 5.6 and 6.3 otherwise.

photoman022 Loc: Manchester CT USA

I use Tamron lens on my D3100 and D3200 and they are very sharp. They were also considerably le$$ expensive than their Nikon competitors.

amfoto1 Loc: San Jose, Calif. USA

There is only one third stop difference between f/6.3 and f/5.6.... so it will be virtually indistinguishable to anyone... pro, amateur or rank beginner.

For a long time f/5.6 was the limit of what would be able to autofocus. Third party lens manufacturers found a way to "fool the camera" into focusing with f/6.3. Today it's fairly common for cameras' AF systems to be able to autofocus f/8 lenses (which largely means an f/5.6 lens with a 1.4X teleconverter).

BTW, most "pros" wouldn't touch an 18-200mm lens. 10X (and more) zooms simply don't have the image quality and general performance a pro would typically require. Most "pros" using an APS-C format camera would be more likely to carry a set of lenses such as 17-55mm and 70-200mm instead (3X or 4X zooms = higher quality images and better overall performance.... plus often feature faster, non-variable apertures such as f/4 or f/2.8).

Okay, sometimes a 10X zoom may be necessary... say you're traveling cross country by bicycle or motorcycle... Or you're shooting from inside a small airplane or otherwise very restricted in what you can carry or ability to change lenses.

You can compare magnified test shots done with each of those lenses here:

https://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=667&Camera=614&Sample=0&FLI=2&API=0&LensComp=946&CameraComp=736&SampleComp=0&FLIComp=2&APIComp=0

You can use the pull down menus to compare with other lenses, zoomed to some different focal lengths and at different apertures. You may also be able to change the camera... if given a choice and able to choose the same camera for both, do that. If unable to use same camera for both, try to keep to two cameras with similar resolution for lens comparisons.

To me the Nikkor looks better... Still, compare any of those "convenience" zooms to alternatives such as a 17-55 and 70-200 combo.

nadelewitz Loc: Ithaca NY

With negative films, a half-stop difference (f5.6 to f/6.3) in exposure is not highly significant...well within the exposure latitude of the film. Transparency film have much less exposure latitude for good exposure, but a half-stop still isn't much.

sandiegosteve Loc: San Diego, CA

The 1/3 stop isn't the whole difference.

The comparison is looking at different AF motors, internal parts and glass. For many lenses, they are sharpest stopped down a bit. I've found that my Nikon glass is pretty good wide open and some of my third party lenses need a full stop to get full sharpness.

From a light perspective, I doubt you will notice a difference. Sharpness? That may depend on the model you get. Some Nikons are softer than others, but generally they are consistent. Third party lenses have a history of having a bigger variation within the same model.

That said, I think if you wallet is happy with it, you will be too.

nadelewitz Loc: Ithaca NY

The OP was asking specifically about f/stop difference. All other properties are subjective as far as what's "good" for you.

frankraney Loc: Clovis, Ca.

If you only need to save for several months, then same and get OEM. Tamron is ok, but OEM is better.

nadelewitz Loc: Ithaca NY

Can you define "better"?

Lens "quality" to a user is a subjective thing. You can QUANTify a lens's properties and function in a test lab, but you can't QUALify a lens as better for me or anyone else. Test lab specs do not necessarily translate to what you or I SEE in our photographs.

frankraney Loc: Clovis, Ca.

You are right, it's subjective, I'm Just speaking from my experience....Tamron for me has not been as sharp as Nikon, on two different cameras.

Other people have had better experience.

JohnR Loc: The Gates of Hell

I would certainly agree that Nikon professional quality lenses, are probably as you say, custom-tuned however the consumer level 18-200 lenses being considered by bellgamin would not be so well manufactured I suspect. Many many people swear by their Sigma and Tamron lenses.

bellgamin Loc: Ewa Beach, Hawaii

I have learned a LOT from this thread. I am a hog lover... suuu-eeeeeeeeeee! Pig! Pig! Pig!

Based on many of your comments, I have decided (a) to go Nikkor, & (b) back-away a bit from 10X zooms. Nikon makes both 18-135mm & 18-140mm -- I figure going for 1 of those 2.

Please let me have your comments.
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P.S. I wonder why Nikon offers 2 different lenses with such near-identical ranges?