I have taken bulls with the 338 WM, 300 WSM, 30-06, 308, 7-08, and 270 Win. I have seen them taken with the 300 WM, 7mm RM, 7mm Wby, 270, 243, etc., etc.
As a backpack hunter I prefer a light rifle (sub 7lbs with scope, sling and rounds in the magazine). Heck, as a day-hiking hunter I prefer a light rifle! Anymore, I use light rifles in 308 Win, 270 Win and 6.5 CM. Despite being a bit of a rifle nut, I no longer own any magnum rifles, having had everything from the 270 WSM up to the 375 H&H.
I think the biggest mistake guys make on first-time elk hunts is going to a heavier and/or harder recoiling rifle (ie, a magnum). You don't need a magnum, and it really should be a heavier rifle than necessary to mitigate recoil. A light rifle is a blessing in elk country.
My advice to the OP is stay away from a 7mm RM or 300 RM. I think the smartest thing you could do is get a lightweight 270 to match your midweight 270. It will be familiar and comfortable. I've seen more elk taken with the 270 than with any other round, and mostly with 130 gr. el-cheapo factory ammo. When anyone new to elk hunting here in Montana asks me for a rifle reccomendation I tell them to get a Tikka T3 in 270. Put a Burris 3-9 with a Ballistic Plex on top in Talley lightweight rings. Add a Butler Creek Mountain Sling, and head into the mountains.