for me knowing your computer's RAM and HDD is not enough you must tell me what is the system's CPU? Did you installed Vmware tools on both virtual systems?

i think Vmware needs more system resources than Vbox and as you have windows seven its better to dedicate 2GB of ram to your real PC it means 1GB for backtrack and 1GB for XP

i extremely suggest you to use VMWare 8 and i dedicated 512mb OF ram for Backtrack and 256 MB for XP i dont have any problem about performance.

and VMware 7.1 is much more better than 7. If you coouldnt resolve the problem disable you AV before you start the VMs and its better to run Game booster programs like Game Fire (its great)

I would suggest to drop VMWARE and switch back to Oracle VirtualBox.

You can run without any problems the vmware image files that BackTrack released on VirtualBox, I'm doing so for weeks without any problems.

Hope that helps. Most people don't know you can run VMWARE images on VirtualBox.

Oh sorry about that, systems CPU is Intel core i5 2430M CPU @2.40 GHz, Yes i installed vmware tools, i never forget that part.

ok man u have good hardware so just change the VM's RAM as i said. and if u still have the problem try to dedicate 1 or 2 of ur cpu's core to backtrack. Again if u failed update ur vmware or install gamefire to boost ur system's performance. Even if u have too reinstall win7!!! I extremely recommand u to use vmware

with the 4GBs of RAM on the system, there is a good chance you are maxing out.  CPU should be sufficient.  If you system is able to I would bump the RAM to at least 8GB.  If you are running heavy scans against your victim VM, keep in mind you are pushing your system into double duty.  BT is going to use the CPU/RAM to run the Nessus scans and your victim VM is going to have to process the network traffic.  Which of course is all happening on the same host. 

As for which is better, well I can never remember why, but I always find myself going back to vmware from vbox.  The major pro for vbox is that it is free, supports snapshots and can easily import vmware images.  VMWare workstation is certainly more expensive, but does come with some nice features.  I am a fan of Unity, and I make use of it when running something like IDA Pro where pushing the view to a full screen display but allows me to run my apps from my host along side it, comes in pretty handy.  I think it also allows you to convert your host to a VM which is nifty.  If you want a true comparison though, I would use vmware player vs vbox.  Vbox will win that one hands down.

Running BT with NAT under a virtual machine is a very bad idea imho anyway.

Your post suggests that there is no bridging in VBOX or that the bridging in VBOX is not working correctly.

Both very untrue, I use bridging in VirtualBox since years without any problems.