Stewart, and not because I'm a homer...
Past 3 year stats...
Stewart - 1800 AB, .313 AVG, .370 OBP, .474 SLG, avg. 21 SB, 16 HR, 103 Runs, 58 RBIs
Lofton had less at bats, so I'll modify him up to match the 1800 ABs
- 1800 AB, .267 AVG, .348 OBP, .411 SLG, averages 28 SB, 15 HRs, 111 runs, 72 RBIs
Overall, you may get a couple more RBIs and SBs with Lofton, but you're giving up On Base which is critical. Also, Stewart plays on a much better (offensivley) team than Lofton, and isn't batting behind a pitcher, so should have more opportunity for RBIs.
Last thing: OPS for the past 3 years:
Stewart: .881, .831, .813
Lofton: .791, .720, .764
Both going down, but Lofton has always been lower. I take OPS to be the "bible" truth-teller on who to start (and who is better).
Take that for what it's worth.