Already owning the 338wm, I'd say one of two options make the most sense:

1) Buy the .300wm as backup to stand in for your 338wm, if that makes any sense to you.

Or

2) Get the 7rm to get something significantly different than what you currently have.

Personally, I have and use both, and wouldn't find myself without either. If I could only have one and I went PURELY by objective logic, I'd only have a 7rm. If I go by purely subjective gut preference, I'd only have a 300.

They're both right answers to the same questions - there are very few applications where one is a right answer and one is wrong. But since you already have a heavy hitting 338, I'd lean towards option 2 above and find myself in a 7rm for a flatter trajectory and a very different weight class of bullets.