I've been at this painting lark long enough, I think, to begin to appreciate the merits of both. Some of it was definitely driven home this past week finishing off both some metal Battlefront 15mms and some plastic ones from the Open Fire box.
The obvious downside of metals is price - certainly in 28mm, metals kick in around the £1.50 a figure mark, whereas plastics by the box work out under £1 a figure (we're talking foot here - 40 Russians in plastic from Warlord = £28, or 57 Russians from PSC for £18.50, or 31 for £14 or so from Wargames Factory - big range in prices there!). In 15mm the PSC US Infantry box is 145 figures for £18.50, the BF metal US Infantry box 122 figures for £35 (and the Forged In Battle metal US company mega deal or the equivalent from Command Decision or Peter Pig are all about the same price).
Having said that, it does look as though certain manufacturers are placing a bit of a premium on their plastics - compare the PSC and Warlord/Bolt Action Russians.
Economies of scale favour plastic - after the one-off five figure sum for preparing the mould (and tying up Renedra's production line!), the unit cost is pretty low, so if you can reliably guarantee a big enough run, plastics are cheaper. Which of course means that generally the target for plastics has to be popular ranges.
Conversely, the tooling cost for metal is considerably less to get started, hence it's possible to knock up small runs of more obscure things (you try finding a German tank killer squad in plastic, for example!).
However, the bigger issue, if you can afford either, is that the moulding process places certain restrictions on what you can and can't do with plastics. This means, pretty instantly, as soon as you want to get clever with your figure poses, your figures become multi-part, or wind up (like one of the Open Fire PaK40 crew I was painting yesterday) with an undesirable moulding artefact that isn't really part of the figure and makes painting harder. But… given the design process usually starts with three-ups (or CAD, more and more), you can get finer detail on plastics, which I think is more of a benefit in 28mm than 15mm.
Of course, as soon as you go multi-part, one of the advantages of plastics starts to wear away, depending on what value you put on your time: I know how long it took me to assemble 72 horse archers for my Parthians...
With well-sculpted metals, though, the world is your oyster. But the sculpting is the key - if, for example, you look at some reviews of BF metals, some are really awful, with no depth. Contrast with the (fairly recent, IIRC) BF British I just painted, which have just the right amount of slightly exaggerated depth (you do, I think, need to overdo it a bit in 15mm) to allow ink-shading to work.
15mm? Metals. Apart from anything else, assembling multipart 15mms is a pain, and in general I find the detail on the metal ranges I choose better that (say) the BF plastics. I may revisit this if/when I get to look at some PSC plastics. For vehicles? Plastic: those really are much cheaper in plastic than lumps of white-metal or resin.
28mm? Torn. To pay for? Plastics. To paint? Tossup. To prepare? Metals, unless someone's getting overly clever with multipart metal figures, then I find plastics easier to glue. My limit's fixing on a shield and wire spear - any more and I'll go for plastic.