It is all relative. But I will take a stab:
Targeted traffic sounds the most appealing. If you have the traffic, then you can A/B test and analyze different metrics to optimize your conversions.
Better Conversions is great, but it depends on what the number they are "better than"
Long term effects on Sales it is difficult to understand exactly what you mean by this?
Either way, your product need not be "blogable" for you to blog. Try to determine what industry will likely use your product and blog about topics that are relevant to that industry.