(In my expierence) The problem is not in the adapters. The problem is inherent to newer Dell notebook designs. I have an Intel 6235 in my old Studio 1558 which gets flawless incredibly fast connection and strong signal for the past 3 years. But with newer notebooks from Dell (such as the XPS 15, etc...) the signal and connection speed are horrible. I began discussing and troubleshooting the issue with Dell support techs going all the way back to June 2012 when I purchased my first 15z and July 2012 when I purchased my first XPS 15 (new generation).
In all, I own and have experience with Dell 17r, 15r, 15r SE, 15z, and NEW XPS 15, and I can say this with confidence:
The wireless adapter is not the problem. I get 25 megabit download speed on all models before the new XPS 15 and 15z. On the new XPS 15 with same adapters I get 5 to 10 megabit if I'm lucky. Same adapter series, same drivers, only difference is the new laptop designs. On the older ones with the plastic case I get fast speeds and great signal on WiFi. On the newer ones with the sleek aluminum case I get terribly slow speeds and connectivity (especially the XPS 15). As beautiful and sleek as the new XPS models are, they have an inherent problem in the engineering and design - to the point where if you rely on fast connection speeds you will drive yourself crazy trying to figure out what is wrong. And that's because the notebook / laptop design itself, NOT the adapter, is the problem, and Dell techs are trained to tell you otherwise.
Want proof? I have it. I can't post it here without my post being deleted. But what you can do is a Google search and a Youtube search, and what you will find is that this has been an ongoing issue that goes back to early summer of 2012, and you will also find that a Dell employee admitted to it all publicly on another forum (that I'm not allowed to mention here) suddenly became no longer employed by Dell after dealing with this issue honestly and openly in public. The rest of that story is up to your imagination.
I'm not saying that Dell is a bad company or that all their computers are bad. I've been a Dell user for a LONG time and own many Dell systems that have no problems. But when it comes to the WiFi speeds on the newer models you will find nothing but dead-ends, excuses, and run-arounds with Dell support. They have a few really good, honest, competent technicians in the higher levels that are well aware of this WiFi "blight", but the chances of you getting routed to those few techs are slim unless you're a big customer. And even then, they have no solutions for you.
Case in point - does this sound familiar to you? - "We will be releasing a fix soon, maybe as soon as 2 weeks".
Yeah, that's what they told me in July 2012.
It is now January 2013.
Just sayin' ....