As I understand it, you want the highest bioavailability possible. Your body will pass a water/SN solution to the small intestine where it can be absorbed very quickly. Capsules could dissolve at different rates, making the absorption process more spread out over time. It also may have the effect of more stomach acid coming into contact with more SN, converting it from nitrite to nitrate before your small intestine has a chance to absorb it. SN kills by converting hemoglobin to methemoglobin, a protein which cannot deliver oxygen to organ cells. The body is very efficient at converting m.globin back to h.globin, which is why its important to absorb enough SN to overwhelm this defensive response.
Basically, the more SN you absorb and the quicker you do it, the higher your chances of successfully ctb. A simple water/SN solution is the easiest and best way to accomplish it. Capsules may prevent vomiting due to the taste/gag reflex, but it seems like a nausea response is the body's defense against an ingested poison, whether you taste it or not. So (in my opinion ONLY), taking it in capsule form probably isn't effective at preventing vomiting, and adds a layer of uncertainty to the process. I am NOT an expert, this is only my personal opinion. Always research for yourself to form your own =)