Which is best?
The answer is that there isn't any best. It comes down to a subjective choice of what the individual modeller prefers. At my local (independent) store some preferred plastics while here at Artificer's Workshop metal minis are the preferred option. In a sense this post is academic as metal is on the way out as far as games workshop is concerned. And to be fair and impartial the quality of the latest plastic releases have been superb. No doubt everyone has their own preferences but here the plastic Grey Knight Terminators are recognised as superb.
(Why does blogger insist on american spelling when american english is more and more infiltrated by britishisms these days - a bit like the french hatred for franglais, see recognised. By the way I know the answer - rhetorical question directed at blogger.)
Back to topic. The future as far as metal goes is undoubtedly finecast. Here despite reservations identified in previous blogs it is recognised (that "misspelled" word again) that this is the future. Current problems will doubtless be ironed out.
The question posed on this blog is really what do you prefer to play with on the table top. The answer here is metal minis - hence the focus on Sister of Battle and Steel Legion and Bob Olley Rogue Trader era minis. Why because they are heavier and subjectively seem more substantial when placed on the table. Plastic minis here and there are definitely plenty of them often have a coin glued inside their GW base to give more weight when deploying on the table. Does anyone else do this? Does it produce more wins on the table top - probably. Try and see.