I do not think that any of the zooms you mention would be materially better than the 18-140, except for f/2.8, which you have the 35/1.8 to cover.
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I'll have to contradict that a little, I'm afraid.
I don't have the 18-140 specifically, but I do have the older 18-105VR lens.
I thought about getting the 18-140 as a cheap update for the 18-105, but found it had the same issue as the 18-105 .. mainly that it loses a lot in transmission when compared to a better quality lens, for example like the Tamron 24-70 VC.
I don't know if it's due to the Fx frame coverage(ie. less vignetting from the larger frame coverage) or just better optic design, but the consumer zooms seem to lose about 0.5 - 0.7 Ev in transmission for the same aperture settings, compared to the 24-70/2.8
I only tested at f/5.6 and f/8 at 24mm and 70mm on both lenses.
From that I believe that a lens like the Tammy 24-70 VC will definitely become a step up in IQ, coupled with the f/2.8 aperture option will be an effective use of limited funds.
A massive downside to this lens tho will be the size of the lens. 84mm front filter threads, makes filtration such as polariser a very expensive proposition(if quality is considered!!). And it's so much heavier than the consumer zoom.
On a D7100 this will have an definite impact on 'handling'. I have a D70s for comparative purposes and it does feel a bit more cumbersome having the Tamron as opposed to the consumer zoom attached.
On the D300(heavier body) with a much more ergonomic grip the impact is less obvious, on the D800 the size and weight of the camera lessens the impact even more.
I haven't held a D7100 since they first came to market years back, but as I remember, it's weight/balance was more D70s like rather than D300 like.
The 2x faster aperture is not only good for additional light capture ability, but at about the 2m focus range(ie. at about where you may want to shoot portraits and a 70mm setting), the difference in aperture ability is definitely noticeable, so for closer in shots you have more background blur scope. While you'd have 140mm on the consumer zoom, at that focal length, you're still at f/5.6 .. and as you have the 105VR lens this one would be the preferred choice for portrait at that focal length anyhow(and it produces what I think is about as good bokeh as any lens can produce!!)
At farther out focus distances, I doubt bokeh and blur options become a major factor. And the bokeh of a 24-70/2.8 is at least an order of magnitude better than a consumer zoom blur quality(aperture design I guess).
While the VC on the Tamron is handy and usable, and the reason I updated to it .. don't discount the older Tammy 28-75/2.8 lens as an option either. (which is what I updated from)
Much cheaper, much more manageable in terms of size weight .. and far far cheaper! But if VC is a consideration(as it was for me) the price difference is worth the effort.
Overall, I'm disagreeing with Les Olsons summary and believe that a 24-70/2.8 lens acquisition will constitute a major upgrade choice from a consumer lens such as the one you have.
