I think Yossu has a point, but the novelty collectability (pokémon effect?) of the biodomes is a winner for me. Where are you finding neps for £4 though?
Plus, they grow like weeds compared to most other Neps I've grown (and killed ). My Windowsill Suki is putting out its second new pitcher and has grown out the top of the dome already in just 4 weeks.
FWIW, of the three varieties I have, Suki is by far the most vigorous, Lady luck the least.