I'm completely speculating here, but I'd bet its only the housing that's brass and the impeller assembly is still plastic. If anyone cares to correct me here, please do. Point being you might be able to replace parts instead of the entire assembly down the road if you have issues, but the max temp and flow rate would not be different. As you already mentioned, I'd assume its all about durability.
Don't want to steal the thread here but I found a March 893-002-02 on ebay for around $25.00. I've been looking to buy a pump and was wondering what the thoughts of the group were? I see from above that the 890 should run about $90 on ebay but I'm finding them for around $130. Would it suffice for the set-up you currently have?