Well, they have the same coverstock and the same finish (judging form the FCC scores), so they are probably incredibly close. The only difference is the Raw Hammer Assault Core vs the Vibe Symmetric ("hourglass") core.
Burn 15# - RG (2.48) Diff (0.050)
Midnight 15# - RG (2.51) Diff (.042)
Honestly, if you already own the Burn you probably won't be picking up a Midnight.