Like Ken said, it's personal preference.

I used mostly Omega (until I got a sweet deal on 8 gallons of Wildcat). I'd occasionally get some Coolpower instead when the LHS was out of Omega. Mostly you can't tell the differenece in the engine running.

Caster does have a higher flashpoint than the synthetics, so it will give just a bit of extra protection if you lean the engine out too much. However, caster also causes a varnish buildup in the engine and makes more of a mess. It can also do funny things to an engine in storage.