I suppose it depends how die-hard and committed to the company you are.
Ignoring the small issue of best value, bang for buck and such, if you really want to give to the company, you'll take the SPP. Why?
You guys have touched on this, but if you want Neptune to directly benefit from your shareholding, you'll give them your money rather than through an on-market transaction where the ASX/Commsec/whoever and the buyers/seller themselves get your money.
Is anyone that die-hard? Quite possibly. Small holders must also consider the "free" transaction that they can undertake by purchasing through the SPP offer rather than on-market.
NMS Price at posting:
22.0¢ Sentiment: Buy Disclosure: Held