Shawn Marion is a much better offensive player as a power forward.
He is a much tougher matchup at that position due to his elite small forward type explosive athleticism + the perimeter skills (jump-shooting, ball-handling, slashing) that were fairly mediocre for a SF where well above average for the PF position.
Shawn Marion's scoring totals are pretty good at SF but his scoring efficiency was also mediocre to below average. In that 51-54% true shooting percentage range.
In this fantasy game, Shawn Marion, the small forward, should always be the fourth or fifth option. In starters minutes, a 12-14ppg type scorer mainly of hustle baskets (transition, offensive rebounds, drive and kicks/dumps, cuts to the rim + the odd one-on-one drive here and there). That is fine, his main value is his elite level defense + rebounding at the SF position.
Edit: Marion had the elite athleticism to be a top scorer at the SF position but he lacked the skill-level. Too average skill-wise, offensively, relative to the position. Why he wasn't a high quality scorer.