Sure, budget is always an issue on the short term, but on the longer term one will benefit from having a better lens.
Doesn't this depend on one's definition of 'better'. The better lens is not necessarily the sharper lens, but the lens which one uses more often, perhaps because of its image stabilisation, its range of focal lengths, its light weight and its general convenience, all of which in total may be more significant than a modest increase in the sharpness of an alternative option.
Of course, I would agree if one has a choice of two lenses which are similar in terms of features such as focal length, FL range, and weight etc, then it would be advisable to spend a bit more money on the sharper lens, whether it's a prime lens or a zoom lens that one is considering purchasing, provided the increase in price is not too ridiculous.
In my view, the ideal lens is a zoom lens which is as sharp, or at least nearly as sharp
, as most good primes at focal lengths within the zoom's range. Such a lens is the Nikkor 14-24/F2.8. This lens is the reason I switched from the Canon system I'd been using for the previous 10 years, to Nikon. If Nikon had not produced this lens, I would have upgraded my 5D to a 5D2, and my 50D to a 7D, and would then possibly have felt too trapped to make the switch to Nikon when later discovering the significant DR advantages of the current Nikon sensors and the attractive performance of the D800E.
However, the main point I'm trying to make here is that I didn't buy that Nikkor 14-24 just because it is such a fine lens. I bought it because I knew I would be using it frequently, because of the history of my usage of the Sigma 15-30mm with the 5D.
Furthermore, I bought it also because on some occasions I'd experienced disappointed with the performance of the Sigma 15-30, particularly at the edges of the frame. I didn't buy the Nikkor just because of a theoretical awareness that it was a better lens than the Sigma.
Another reason for not spending significantly more money on a sharper lens, which will undoubtedly produce a greater increase in detail when later coupled with higher-resolving sensors, is that one can't predict what improvements in lens features and FL range will be offered in the future.
For example, the Canon 70-200/F2.8 and 70-200/F4 had fine reputations for being sharp lenses for many years. Some folks preferred the 70-200/F4 because it was lighter and less expensive, yet almost as sharp as the F2.8 version, but these lenses didn't use to have IS. The F2.8 version was the first to be upgraded with IS, then the F4 version sometime later. Now Nikon have come out with a VR version of the 70-200/F4.
Just because a lens is particularly sharp doesn't mean you will be satisfied with it for ever more. The main criterion in my view is to get a lens which you know you will use because it has the ergonomics and features, and the range of focal lengths which are most useful for your style of photography. I think one can be too easily seduced by the glowing reports of the sharpness of a lens, buy it, then find that one hardly ever uses it, except perhaps when comparing the performance of different sensors.
I think the last time I used my Canon 50/F1.4 prime was to compare the performance of a newly purchased Canon 50D with an older 40D, photographing the Australian $50 banknote taped to the wall.
As I said, in a better lens the overall image quality is likely to be better (CA, glare resistance, bokeh, resolution).
Glare was occasionally a problem with the Sigma 15-30, but only when pointing the lens in the general direction of the sun, which one tries to avoid doing whatever the lens. CA, barrel distortion etc, can now be fixed automatically in ACR or DXO Optics, so one need not place too much significance on such defects unless they are really bad.
I have used more (increasingly better) sensors per lens than the other way around. That's because my lens choice has always been for the better quality alternatives at that time. Good lenses last a long time, so saving a bit longer for a better lens will pay off in the end.
So have I, yet I've never bought any really expensive, first rate primes, and those which were moderately expensive, such as the Canon 50/F1.4 and the Canon 90/F2.8 TSE, I have not used much simply due to the general inconvenience of a single FL.
Whatever the quality of one's lenses, one will appreciate some degree of improvement as sensors become higher resolving, unless the lens is really bad. My only superzoom is the Canon 100-400/F4.5-5.6 IS which I bought with my very first DSLR, the 6mp D60, about 10 years ago. There is no doubt whatsoever that the 15mp 50D produces sharper images with this lens, than the D60 ever did, yet this is not a particularly sharp lens. At 400mm it's sharpest at F8. For me, the alternatives are too heavy, too expensive and too restrictive if they have a fixed FL.