Back when I was involved with 3d applications we found that there wasn't much difference between the high end gaming cards and the lower end workstation ones. I suspect that this hasn't changed much.
The main difference these days seems to be the type and quality of the outputs. A workstation card is more likely to have twin DVI outputs whereas a typical gaming card will still have one DVI and one VGA output (for example). As @davr points out the high end gaming cards will have twin DVI outputs making the differences between the types of card even smaller.
Presumably you've checked the specification of each card? Do the benchmarks they use match what your application does?
It would be nice if you could try the card out and run some benchmarks. Do you have a friendly local store you could persuade to set up two machines (or swap cards in one machine) and run a benchmark with your software? If they knew there was a sale in at the end of the day they might be amenable.