Jack,
I used a Tamron 18-250 as my only lens for the first 6 years after getting my first DSLR (K100D Super). After 20+ years of swapping between two zooms on my previous film SLR, the versatility was invaluable. No doubt you had the same experience with the Sigma 18-250. All the comparisons I have seen between superzooms and kit lenses (which generally have similarly slow maximum apertures) put the superzooms ahead of the kit lenses for image quality. And of course the superzooms offer more reach than the kit lenses.
So it seems to me that the only downsides of preferring a superzoom to the two kit lenses are:
1. The superzoom will be bigger and heavier than either of the kit lenses individually (although probably less than the two combined)
2. The kit lenses are now WR.
3. The kit lens might have QuickShift?
4. The kit lenses would support in-camera correction of jpgs for distortion, CA, vignetting etc. Third-party branded superzooms won't.
5. The superzooms are more expensive.
But for wildlife I found that I wanted more reach than I could get with the 18-250. I had a Sigma 170-500 for a while, and now I have a Pentax DA-L 55-300 and a Sigma 400mm f5.6 Tele Macro. These are much better for birds and other wildlife than the 18-250.
I guess you are wondering about the new Sigma 18-300 as a possible compromise? Remains to be seen how it compares with the 55-300 at the long end:
www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/314783-sigma-18-300-review.html
Only a couple of user reviews so far:
Sigma 18-300mm F3.5-6.3 DC Macro HSM (Contemporary) Lens Reviews - Sigma Lenses - Pentax Lens Review Database
I'd say from my experience with the 18-250 that the 55-300 is likely to give much better results at the long end for two reasons:
1. The slower maximum aperture of the 18-300 (f6.3 v f5.8) limits its use to when the light is quite good. AF can be marginal at f6.3 in mediocre light. Also if the light is not great you would need to lower the shutter speed and/or raise ISO to what might be unacceptable levels. To make matters worse, these wide-ranging zooms usually need to be stopped down for greater sharpness. Take the aperture to f8 and the low-light issues are increased.
2. The 18-300 is an internal focus lens, but the 55-300 is not. The 18-300 will only provide 300mm-level magnification (if at all) when focused to infinity (this is called focus breathing - see numerous threads about this). If your subject is say 5-10 metres away (say 15-30') the real magnification will be equivalent to much less than 300mm. With the 55-300, 300 means 300. Even at 250mm, the 55-300 will give much more magnification (ie narrower field of view) than the Tamron 18-250 at 250mm except with far distant subjects.
I have now finished up back with a two-zoom setup for wide-long that I left behind with my film SLR. I now have the DA 18-135 and DA-L 55-300. Each provides a significant step up in image quality from the superzoom. Each lens is about a stop faster than the superzoom.
I just love the quiet AF, WR and QS of the 18-135, it is compact and light weight, and the IQ is very good for what it is (particularly at the wide-normal end). And of course its a very versatile range (equivalent to the whole coverage of the 28-85 and 70-210 zooms I had on my film camera).
The DA 16-85 and Sigma 17-70 C are also worth careful consideration as alternatives to the 18-135. Some people find the extra 1-2mm on the wide end more useful than the shorter telephoto range.
The next step up from there in wide-normal zooms is going to one of the f2.8 zooms like the Tamron 17-50 or the Pentax DA*16-50.
My version of the 55-300 is the poverty pack one (plastic mount, no supplied lens hood, no QS, not WR); it's amazingly cheap, but if you can afford the current HD WR version (it was going for around $US260 recently) it would be a better option.
The next step up from the 55-300 is the DA*60-250 f4 or one of the 300mm primes (either can be paired with a teleconverter for more reach), or (for more reach) the Sigma 50-500 (Bigma) or 150-500, or the Pentax DFA 150-450. All much heavier and much more expensive. But if you really want long, or you shoot in low light, they will give you significantly more than the 55-300.
I like shooting nature shots mostly. If all my gear were stolen and I got a decent insurance payout, I think I would get the DA 16-85 and either the DA*60-250, DA*300 or F/FA*300 f4.5 and DA WR teleconverter (plus 12-24, 31, 43, 77 and 100). I could certainly live with just a 300 prime + TC for the long end - they aren't too heavy, the IQ is stellar, and I find I mostly use the 55-300 at 300 anyway.
You will probably have responses on this topic in your other thread by now! Good luck.