I use synthetic simply because it doesn't leave dark varnish and sludge buildup in the engine. An engine thats been maintained with synthetic will have a nice golden/titanium color while dino leaves a dark varnish and sludge buildup in areas.

I don't believe either will make the same engine perform any better but seeing first hand the difference in the long term I opt to use synthetic. For roughly 4 times a year and what amounts to $100 in oil its not really that bad.

Just a matter of prefference and engine cleanliness for me.