Are products labeled Kosher or Halal generally of a better quality than those that aren't?
This probably depends a great deal on regional and political circumstances, but where I live (Toronto), there are dozens of Kosher and Halal grocery stores (or stores with Kosher or Halal sections), and in my experience, the meat is actually generally of worse quality.

Halal, as it applies to meat, refers to which animals are allowed and the required method of slaughter (an incision across the neck, cutting the jugular vein and carotid artery in a single motion - Kashrut requires the esophagus and trachea to be cut as well).  Kosher meat also has be koshered, which means soaking and salting the meat in order to draw out the blood.

As you might expect, the method of slaughter has no tangible effect on the quality of the meat, but the koshering process does, and the effect is not positive - it dries it out and makes it salty, and most kosher meat sold in supermarkets (and even the majority of butchers) is quite tough compared to its non-kosher counterparts.

That said, some people do prefer Kosher (not Halal) meat for health reasons, because the blood may carry uric acid and other compounds that some people do not wish to eat.  I've never seen any specific scientific evidence that these compounds are harmful as far as meat consumption goes, and any reports of this leading to better flavour are anecdotal at best, but I am not an expert on the biological factors, so don't take my word for it.

Various Halal/Kosher laws also govern the facilities in which the meats (or other foods) have to be prepared, but it is important to note that these laws do not actually govern general sanitization!  In fact, a number of butchers have been closed down by municipal health inspectors at various times for unsanitary conditions and other improper food handling practices (such as adding food colouring to meat to make it look fresher).

Note that I am not trying to impugn the Kosher or Halal industries.  These practices are every bit as common in the mainstream food industry.  I am simply saying that being kosher or halal does not have any real preventative value when it comes to shady/sloppy practices.

Another factor to consider is that supermarkets and butchers are a competitive industry, but being subject to Kosher/Halal laws greatly reduces the competition and raises the processing cost by sometimes astonishing amounts.  These simple economic factors would lower the quality of any food being produced because parties collude and cut corners.  Again, I cannot speak for every single region of the world, but here, you cannot find a kosher ribeye or prime rib steak.  They simply do not exist.  Whether it's because the butchers all prefer to make cheaper cuts or because they figure that a choice cut would simply end up being too expensive for anyone to consider buying, I'm not sure.

I know that several TV chefs in recent years have recommended buying kosher meat (and possibly halal, although I haven't personally heard that), but assuming that halal/kosher is optional for you (i.e. you are not Muslim or Jewish) then always do your own research before spending a lot of money on what may often turn out to be an inferior product.