There is a difference between metal and plastic paint tubes, but I don't think it matters as much as whether you're buying artist's or student's quality paints. Especially if you use up your paint fairly quickly, so it's not sitting around in a partly used tube for years and years. Some 'plastic' tubes are in fact made from laminated layers of plastic, metal, and paper -- you can sometimes see the layers at the sealed end.
Metal tubes have the advantage that they keep their shape, whereas plastic ones tend to 'bound back'. This means it's easier to scrape a metal paint tube flat to get the last bits of paint out. A metal tube is also less likely to suck air back into it than a plastic one, which is of course of concern as paints dry on exposure to the air.
I like to keep the paint up at the cap end of a tube; this way it's easy to get a decent squeeze of paint out of a half-empty tube, and it reduces the air that's in the tube. I use the handle of a brush to push the paint forwards in the paint tube and then roll up the back. A metal tube will stay rolled, but a plastic one won't generally without using a clip of some sorts, or at least not until it's quite empty.
I've found both metal and plastic paint tubes stand up well to abuse, such as being squeezed to get the last tiny bit of paint out and travelling. I've never had either break on me.
Mark Golden of the Golden Paints says he is responsible for Golden using metal tubes: "The tube was also my choice. I like a metal tube better than the plastic. There simply tends to be less suck back of air in a metal tube, and less of an opportunity for the product drying in the tube." 1
I have to say I've never bought paint solely because it was in a metal or plastic tube, though only one of my favorite brands of acrylic paint, Liquitex, comes in plastic tubes. (Or to be technically accurate, Liquitex comes in Glaminate, tubes made from laminated layers of plastic, metal, and paper.)
If you often mix up large quantities of a color, some art stores sell empty metal paint tubes, which are ideal for keeping it to use another day. They're unsealed at the end so you can scrape the paint in using a palette knife, and then roll up the paint tube end. Loosen the cap so that air can get out as you put in the paint.
References:
1. Mark Golden on Paint Blog, 20 September 2006, The Lids Off – (Or Not)
