Razorwings probably benefit from black heart and flayed skull most. Black heart makes you a little tougher, while flayed skull lets you ignore cover. Poisoned tongue is handy for the necrotoxin missiles but not really needed, while obsidian rose is probably the least useful since with their speed fliers don't really need the extra range.
You're presenting your dilemma as all or nothing when I don't think it has to be. Take some shredders and blasters in your kabalite units, maybe 50/50 each than take say 2 dissie razorwings and 1 dark lance.
If you're taking 3 fliers, definetly take them in an airwing for the extra cp, but given you already have 3 ravagers I don't particularly think you need 3 razorwings as well.
The Night is Dark and Full of Terrors...