The cash value is about the same, give or take a bit due to local supply and demand. Myself, I dislike and have no use for GLocks and really enjoy and use Vaqueros, so, I would gladly trade any Glock in my posession for a Ruger SA.
It really depends on if you have a desire and a use for the new gun. Obviously, the Glock is better suited to self defense in today's society than the Ruger, at least in most people's opinion. However, one thing a Glock will never be is a .357 magnum revolver. SO, if you have your needs covered, and its just a matter of want, then why not?
Personally, I would never make the trade the other way, give up the Ruger for a Glock, but that's just me, and my personal situation.