AIUI DT and Sapim both use wire to the same specification that comes from reputable steel manufacturers such as Sandvik. Unless they impose an additional layer of materials QA (better) or occasionally buy material from other sources (worse), then there shouldn't be any difference in the material itself in these two spoke brands, only the way it is processed.
Even in 'good' spokes there are small variations in the shape and length of the J bend and this (together with variations in the way hub flanges are made) affects the fit of the spokes into the flange. This usually doesn't greatly affect the longevity of the wheel, provided the wheel is built with the requisite degree of care.
To get the spokes to sit in the flanges reasonably well even if they don't fit perfectly, the J bend needs to be 'set' and then the spokes need to be stress-relieved, e.g. by using a large tension overload. This effectively erases most of the (bending) residual stresses and variations in bending stresses that arise through tension variations in the spoke that occur in service.
Otherwise these bending stresses will locally (at the surface of the spoke) comprise a high mean stress (from residual stresses) plus a larger than normal cyclic (fatigue) element, which damages the spokes more quickly than if effective stress-relief is carried out.
Tension variations in the spokes from (poor) building just add to this, by altering the overload required to promote effective stress relief, as well as altering the cyclic stresses seen in service.
I commonly use spoke washers to improve the fit of spokes into hubs, such that the gap between the ( 'inside' especially) spokes and the flange is minimised. This minimises any cantilever bending in the J bend that might otherwise occur. If the J bend is entirely supported by the flange (which deforms to suit in most cases) this is the best arrangement. Note that this does not mean that you don't have to stress-relieve the spokes, but it does make for a better wheel.
Regarding used wheels which have started to break spokes there is some good news; provided the fit meets some minimum standard, and the spokes are not defective, simply stress-relieving a used wheel properly (which may not have ever been done in most cases) will greatly improve it.
It doesn't entirely eliminate the possibility of further spoke breakages however; some may already be cracked (eg by fatigue) and will now fail in time regardless of what you do. For years no-one knew whether there were likely to be few or many such cracked spokes.
I have tried to answer this question by dismantling scrap wheels in which there have already been some spoke breakages. Replacement spokes are easy to distinguish from original ones in most cases. The old spokes have been removed and examined to see if they are already cracked (at the J bend) or not.
The answer (based on about a hundred wheels and many thousands of spokes examined) seems to be that if you have already broken 'n' spokes in a wheel, it is possible that there are up to 'n' further spokes that are cracked and will break, with an average value of ~'n/2'.
Thus if you have already broken (say) four spokes it is quite likely that you already have two further cracked spokes and if you are unlucky there might be four. These spokes will break eventually. However the remaining spokes will (given the caveats already mentioned) will have a very long life if the wheel is now stress-relieved properly.
Note also that good quality spokes will take being 're-set' a few times without developing nascent cracks. You can verify this by trying to break a spoke by flexing it back and forth at the same point; cheap/bad materials won't withstand many flexings, but better quality spoke materials will.
Thus you can now make a rational choice about whether to
a) carry on or not with a wheel in which you have already broken a few spokes and/or
b) whether to re-use spokes in another build or not.
and so forth.
Note that professional wheelbuilders won't recommend that they do either of these things with a customer's wheels, mainly because they are required to guarantee their work. But that does not mean that you can't build and maintain very reliable wheels for yourself using this information.
Just to add that I have built hundreds certainly if not thousands of wheels over about 35 plus years and (touch wood, to my knowledge, and I do ask folks to definitely tell me if it ever happens) I have yet to hear of a spoken spoke in my new wheel builds, some of which have now done 60000 miles or more.
A final comment is that I most commonly use a variant of Brandt's stress relief technique; instead of squeezing the crossings in the plane of the wheel, I squeeze the crossings towards one another i.e. at ninety degrees to the Brandt squeeze. I think I originally started doing this due to a misunderstanding; however it seems to work at least as well if not better, because it is less likely to leave a bad nipple kink which can happen in some cases otherwise. The J bend may also be better set to suit wheel this way, too.
cheers
