Easy. 4K every time. The slips are nice and I'd rather watch a movie in the best possible format rather than have a package that is slightly more collectible. If the steelbook is a work of art and has some awesome embossing/debossing/gloss, I pick up both. But 4K is much better than a subpar or par steelbook.
I'm there with you:/ I've PO many steels from BB and had to skip some cause they didn't include the 4K discs. Coco and Dunkirk BB steels don't come with 4K but I'm definitely getting COCO steel, looks awesome.