I've got both; they are most definately different inks.
Heart of Darkness has a brownish cast to it, where Old Manhattan is a more 'pure' black. All things told, any difference between the two is small - the non-eel Noodler's blacks are the darkest of all of the black inks I've used (and I have
many blacks). Even the eel blacks are quite dark.
My personal ranking is along the lines of:
1.) Old Manhattan (No cast; just black)
2.) Heart of Darkness (brownish cast)
3.) X-Feather (reddish cast)
4.) Bulletproof Black (no cast, just black)
But the difference between the top 3 are small and debatable. I personally purchased all of 'em. Reasons:
1.) They're all bulletproof
2.) They're all very dark
3.) Love the different labels
4.) I have no issues with flow or feathering - I'll use all of them, and love all of them.