I'd be happy with either one and you can toss Gurley in as well.
Hard to compare because Elliott played on a better team and better offense but he was a complete back with Meyer saying he was the best RB without the ball he'd ever coached, that's high praise. It was hard for defense to key on Elliott because of the other parts of the offense but other than TE, Penn St. only had Barkley.
I watched Barkley play quite a bit and he's a freak. Both Elliott and Gurley were hurdlers so they come by that athleticism honestly and can do things most other backs can't do but Barkley's balance and agility for his size is mind boggling.
I'd take either one but throw in the person side of it and give me Barkley. He was more mature at 16 than Elliott was at 21.
As to which will be better in the NFL, we'll have to wait and see but Elliott got to go to a team with an OL and Barkley gets to go to a team with the best receiving corps in the league and I would bet Barkley's OC will get the ball in his hands via the pass more than the Cowboys did with Elliott. In fact, I am predicting that little Mr. Hates Kicking Nets is going to have a meltdown on the sideline at least once because they got Barkley to lean on and run the offense through him, not through the WR.