if you are in the US you have to pay, which comes out to about 20% pro-rata, on 1256 contracts based on a 60/40 long/short term tax treatment.
cfd's are tax free if you can get access to them abroad.. it depends on your citizenship and tax jurisdiction.
most spreads on cfd's are not 20% bid/ask, so it is worth it especially in fx which are quoted 2-4 pips at some firms. if you are a US citizen - just trade futures, bc you stil have to pay taxes on cfd's.
Appreciate the spirit in which you started this thread but, unless I've misunderstood you, the above logic is quite flawed. % tax on profits is not comparable to % edge loss in bid/ask. In nearly all practical cases, the bid/ask edge loss will dwarf any tax considerations.