I would stick to the cast iron option. Stainless is not that fond of being heated, nor is it that fond of seawater. You will almost certainly find that the cast iron option is thicker too. I got 20 years out of my last Volvo one, even then it was not quite shot when I changed it.
I have not seen the stainless option, but if it is made with a tube with a seam in it, it really is not likely to last long.