Basically kingston will use the most easily available affordable chip and market it under their brand (I think they also buy PCBs, but I'm not sure). Fortunately they don't do any relabeling on the chips, so the is a chance you might find a good chipset when you're looking in shops.

Corsair on the other hand, although they do the same thing they completely relabel their chips. There is NO way to find out if the VS you get is good or bad at OC-you can only guess by OCing it and noting the pattern of voltage/speed combinations you get. However IMHO generally Corsair uses slightly better chips than Kingston.

Summary
Kingston-do your research, and search shop after shop for the chipset(s) you're looking for and you might get some cool OC rams for budget price. Prepare for a LOT of searching.
Corsair-cross your fingers and just buy. This is ram lottery. Hopefully you get a good one.

Corsair relabels all their chipsets, and its not possible to check CVS chipsets even if you're working in Corsair-you can only check for their high end series like XMS and Dominator. ValueSelects chipset information is classified or non-existant, most you can do is check who they are buying from, and hope you get one of the better ones.

Well, its good to know, since if you know the chipset and batch you can easily find out its OC ability even without trying it out. Up to now, I've yet to find out who does Corsair buy its Valueselect chipsets from. All I know is that they buy from more than one chipmaker-Valueselect OC ability is unusually inconsistent to be from a single chipset batch.

Slower timings will usually allow for higher overclocking, though sometimes lower clocks and tighter timings are better. Above DDR800 there is usually minimal increase in real-life performance, so 4-4-4-x is usually a common choice as its not too slow but still allows for adequate OCing with easy to find, but better than average chipsets.

For normal use it doesn't really matter what chip is it, all ram WILL work at the default spec or else you can RMA it. What counts is that you get a spec decent enough for your use and there is good warranty; Kingston and Corsair are good bets.

Its only when overclocking that the chip matters. Lousy chips won't overclock far no matter what you do.

If they're new then you should go back to the shop and demand a replacement or refund. How the hell can a shop sell ppl faulty stuff (unless the buyer us buying with understanding that its faulty)? Not stable at stock means faulty, apa lagi if not working.

And yes, lifetime warranty is pretty much standard these days, there's no reason to skimp and get anything less.