Hi 270797lb, and welcome to Geeks to Go!
Interesting question. The i5 is the more powerful processor, but not by a long shot. It looks like the i3 is running for about $125 on the market right now, with the i5 running at about $190. That's a $65 difference, which is fairly significant. You stand to save $130 if you select the i3. I think there is a lot of room for discussion and different viewpoints here, and I certainly hope we get more input from the community.
The i5 is a quad-core processor whereas the i3 is a dual core. The i3 makes up for it with another two logical cores, but it's not the same (to me) as four physical cores. The cache sizes are also larger on the i5. Having said that, I think the i3 is a better value in terms of price. When we start getting into AutoCad and Photoshop, I believe (though I could be mistaken) that the graphics card is more important as it will act as the processor for these applications. The purpose of those applications centers on image manipulation and 3D imagery, which is a task for the GPU. With that in mind, the i3 may work out just fine, even though it is a slightly less powerful chip.
If I were in your shoes, I think I would consider the i3 unless I found the i5 at a more reasonable price. I would also recommend investing in a quality graphics card since that seems to be the main purpose for these workstations. RAM will also be an important factor to consider.
I would really like to see what input we can gather from the community as well. I see a lot of room for debate and welcome other viewpoints!
Best of luck!