Probably the more expensive model but you've done a poor job giving the necessary details.
Provide links to the laptops or more detailed technical info.
Things like webcam, warranty, etc have nothing to do with your basic question.
AutoCAD performance will be almost identical between the two. AutoCAD will do a lot with 4GB of ram, so you have plenty of ram with either one. Also AutoCAD predominately uses only one core so quad core doesn't help you there.
Rendering and video processing are in general the apps that are most mentioned as benefiting from quad core processors. That doesn't mean that Indigo definitely uses multiple cores but you can probably find out with some research.
Personally I like quad core processors for anyone that uses their machine to do a lot of stuff, but there might be an affect on heat output and battery life in a laptop.
I purchased an XPS15z from Dell (4 actually) and I am sending them back today. I have had nothing but nightmares with troubleshooting the integrated graphics card and the Nvidia 525M. Even though there is an Nvidia control panel that allows you to select which programs should run with the nvidia and which ones should run on the integrated gpu, it doesn't matter! I have been on the phone with Dell (tier 3 still is trying to diagnose - they sent me 2 new ones today but i have decided that i'm done). Nvidia has NO SUPPORT at all. Go to their website and look for the Optimus forum. They have a FEEDBACK sticky and everyone is complaining about this yet no one is responding from Nvidia.
Before i went investigating with benchmarks and the like, i knew something was wrong when i launched autocad and saw it behaving very poorly. Just dragging the cursor across the screen presented shadowing, and horrible regen times and this is only with 2D. I have not even tried to open a 3D drawing with that machine. When you do a 3dconfig, it shows that the card being used is the intel hd 3000 (integrated card on the cpu). No matter what options are induced, the intel card is still the one being used.
Now, Sketchup runs like a champ! I mean it was awesome opening up the 100+MB file and flying around the screen, orbiting etc. without hesitation. Truly impressed, but if Autocad and other programs are so horribly slow without any support for this Optimus, then I can't be their guinea pig.
Sorry for the rant, but i hope this helps someone else out.
Oh, besides the performance end of things for this laptop, the build quality is just terrible too. The first 2 laptops we got were twisted to the point where you had to put a quarter under one of the rubber feet to balance the twist. The chrome bezel is glued on and was already showing signs of coming off. I have had these computers for 14 days. The casing was very thing to the point where i know i couldn't carry this around with one hand. My old precision M90 was a brick and was built to last. I don't get the same feeling about this laptop.