Gurley never ran behind shit offensive lines like what Jackson had. Gurley never had eight consecutive 1,000 yard+ seasons like Jackson did. Jackson may not have been a top ten pick like Gurley was, but in his prime, he was big, had breakaway speed, was impossible to tackle once he got going, and yes, he never got injured like Gurley did. Even when Jackson was injured (Eagles game when Nnamdi Asomugha tackled him after a long touchdown run on the first offensive play, if I recall correctly), he came back when it mattered and still got his 1,000 yards.
And let's not forget his character. Steven Jackson was the absolute heart of the Rams. He never bitched about anything team-related, never sulked about his coache), even when they were absolute shit like Linehan and Spagnuolo. He gave everything and more for a team that flat-out sucked when he played with us.
Gurley sulked, threw Fisher under the bus, played games with McVay, and tapped out of games on specific downs where we needed him. Yes, I loved TG30 when he played for us, but the guy was not a team player around the end of his tenure here.
And yes, I'd go with prime Jackson - a prime Jackson that had nearly 2,500 yards from scrimmage in 2006, his absolute pinnacle - over prime Gurley.