My seat of the pants view is that, costs aside, Kelley and Heilmann are of about equal value.
Kelley's advantages are an overall edge in offense (esp OBP) and an edge in durability: he's bulletproof whereas Heilmann is 600+ PA.
Heilmann's advantages are defensive range and arm. Nobody's ever come up with a way to measure arm value accurately, but experience suggests to me that you can win with a +2 in LF if you have sufficient positives elsewhere. Heck, I've seen teams win a lot of games with Hamilton or Combs (+3) in CF.
So for me, Kelley and Heilmann are roughly even in actual value, and the deciding factor is that Kelley is 1.5M cheaper. If you can find a good use for that 1.5M, I would keep Kelley and invest the extra cash elsewhere, but that's just me.
Anyway, I'm putting my money where my mouth is because I'm starting a new 100M league in Forbes 09 with Kelley in LF and Heilmann out there on waivers. We'll see how it turns out. And good luck with your team.