It depends on your budget. The falcon bow tiered up to tier 5 will be still worse than soulbleeder tier 0. On the other hand, the falcon bow is so cheap you can tier it by fuse (no need to make cheaper items and transfer as it won't pay off).
So in general, if you're unable to invest 300kk+ to buy soulbleeder, just to tier up the falcon bow to tier 1 might be an option just to enable onslaught as the cost is approx 2 times the price of falcon bow + 4kk for fuse and extra exaltation cores (so should be around 45-50kk for falcon bow tier 1 on most of the worlds).
It might be hard to sell that bow later though if you decide on buying soulbleeder :-).
Also, please take a notice that 1 tier on a weapon will give you approximately the equivalence of 1 skill, so if your skills are cheaper than tier, then go for skills first.