What velocity are you running with the two? A couple of minutes with ballistic software should do the trick for you. The 200 is going to retain more energy @ distance than the 180 due to the increased BC of the weight AND the boat tail. My experience has been that flat base bullets tend to be easier to "tune" than boat tails; meaning they aren't as finicky & tend to print somewhat smaller groups. Some say it's due to consistent pressure on the base of the bullet?? I'm no expert, that's just something i've stumbled acrossed. Which bullet shoots better for you? At 300 yards, I probaby wouldn't worry too much about which is "better" as the difference should be slim. At 500 however, especially with the 350rm you should finally start to see the difference in drop/drift. Albeit, I have no idea what the BC's are with either bullet you mentioned... the 350 isn't "considered" a long range caliber, but it should still put plenty of hurt on a deer @ 500yds....
Just my thought .002.gun)
BTW- I've got a 358STA collecting dust in the safe... maybe I should break it out one of these days...