I'm assuming you are a beginner with a 30 size ABC engine here.

Castor oil can withstand higher temperatures than most of the synthetics. This give you a bit more margin for error if you set the mixture too lean. It also manages to stay coated on the guts of an engine better, which helps prevent internal rusting.

On the downside, castor leaves the muffler and cylinder coated in brown gunk, which bakes on and is very hard to remove. One engine I had needed to be disassembled every few gallons to remove the gunk from the cylinder. Ringed engines can get the ring stuck in the groove if you don't ensure the the gunk is removed by using after-run oil.

Used regularly, and run dry after use, an engine won't rust if the all synthetic oils are used. For long-term storage, after-run oil may be necessary, however. I have plenty of engines that have never seen castor in their life, and they just keep going until they wear out.

In you case, I'd go with the castor blend, just because it's easier to get. I don't mind having brown, gunky mufflers, and the cleaning out is pretty easy if your engine is easy to remove. You just pop the head, pull out the liner and shin it up with a piece of Scotchbrite.