Let me start by saying they are both great guns, nothing wrong with either.
While I have several S&W autos, I don't have a 4586. I do have an early 90's West German P220. I sure wouldn't trade it for a 4586.
As stated above, the Sig will hold it's value better, might even appreciate more and quicker. It will also be much easier to sell if and when that time comes. Accessories (if you don't have them already) will be easier to find and there is a larger variety of them made for the sig.
The real answer is of course to keep them both. I have never regretted keeping a gun I considered selling, but I have regretted parting with many that I have sold.