Completely different beasts, good for different purposes. Solo = cool vapor, great taste, little or no smell, longer sessions (slower draw), not real portable due to size and shape. PNP = hot vapor, degraded taste after 1-2 draws, big clouds, prominent smell when heating, can be used with a much faster draw, very portable due to compact size and slim shape. Really your question should include what you want from a vape to begin with. PNP is better for some things, Solo is better for others.