I haven't used either of these knives, but for lack of more experienced user's input, I'll tell you what little I do know.
Of course, both of these knives have superb reputations, perhaps the military more than the gravitator, so it would be very important to know what you need from these.
First of all, I think the size of knife you want is going to be a pretty important factor. For instance, the gravitator is more than half an inch shorter in blade length, and almost an inch shorter in length closed.
For me personally, a four inch blade is too long for an every day carry. For one, I simply don't have any need for it, but you can imagine that a person in my environment of either college or the hospital might offend with a huge "mean" knife.
Another factor in EDC is the styling. Spydercos have a tendency to go understated, which, in public places, is a positive thing. The gravitator, however, is a pretty extreme looking knife. I took a glance at one today, and I can imagine that the sheeple might be quite offended by its presence. I personally find the gravitator too extreme for me.
You've probably noticed by now, but both use linerlocks (I believe). This might sound strange, but I like my spydercos to have lockbacks, I like my benchmades to have axis locks. This is yet weirder because all of my current EDCs are linerlocks. But, being as I can't afford much from those two, there's a simple explanation. In any case, I'll just have to assume you like the linerlocks.
The price is probably another important difference, the gravitator costing about 50 dollars more for a very similar knife.
If you need a roughly 3.5 + inch long blade, I'd choose the spyderco, though to be honest, spyderco actions have been a real dissapointment for me. I'd still probably choose the spyderco. I prefer the styling and the price enough more to make the difference for me.