I don't want to state this benchmark as definitive, but it is the best data that I could find.
Price Score Card
$99 487 ATI FirePro v3800
$170 687 nVidia Quadro 600
$450 1,104 nVidia Quadro 2000
$160 1,156 ATI FirePro v4800
$385 1,432 ATI FirePro v5800
$780 1,893 nVidia Quadro 4000
$630 2,397 ATI FirePro v7800
As you can see from the first graph, the ATI cards get you better performance/dollar across the board. The $160 ATI FirePro v4800 ($160) actually scores better than the Quadro 2000 ($450). From the second graph, you can see exactly why I like the v4800. It is a good "value".
Benchmarks taken from: http://www.videocardbenchmark.net
A higher number is better.
How reliable are they? Well, it's free information from the internet.
The ATI FirePro v3800 is adequate for small assemblies for the average user. I suggest the ATI FirePro v4800 for most people. It actually may be overkill, but for that price, who cares. I'm sure you can see why I don't suggest nVidia cards anymore.
Other very serious considerations are system stability and compatibility. Many people prefer one brand over the other based on their experience. That may be worth a few hundred dollars, but that is your decision.
The big point that I think needs to be taken into consideration, and probably puts this whole conversation into perspective, is that the Quadro 600, and the ATI FirePro v4800, are both probably more than enough for everyone. So even though some of these higher cards may be of a resonable value, and get a higher benchmark, it is all going to waste. SolidWorks is not super-graphics intensive, and doesn't require a strong-man card. This depends on how you use it, but for people who are not doing massive assemblies, or massivly complex shapes with transparencies, and edges shown, and assemblies, all at the same time, then these cheaper cards are more than enough.