A pastry bag would be better. These guns are tough to use for icing…I find…They are OK for dispensing cookie dough…but for icing you would be better off buying some disposable pastry bags, a plastic coupler and a few basic tips (Open star, flower petal and a couple of plain round tips of different sizes.) It will probably cost much less and you will certainly get better results with icing.
I would buy metal. They give a sharper/cleaner shape when you use them. Also, if you are going to use them for fat based icings (like buttercream) rather than royal icing, they will be easier to clean if they are metal. They last for ever, and quite frankly, you can do a lot with just a few.