In terms of burn speed they're nearly identical. I see way more H1000 in my AO, and I keep a lot of it on hand for .300WM. I only see the pound jars of Retumbo, and I try to only buy jugs. They're both temp stable stick powders (meter like shit). I damn near think it's marketing and they're the same thing, although some may vehemently disagree...
I shoot 230s, which are super heavies out of the .300WM, and I would think you'll get great performance with either @300gr in a .338. What other people experience may or may not be applicable. It seems to me that every rifle is different, and you just sort of have to experiment with powder and seating depth to find a sweet spot.