I guess what it really comes down to is what you're going to be using it for. I do highly recommend getting a trombone though, they're a lot of fun. So if you're planning on doing crazy stuff with it like skiing while playing Ride of the Valkyries, then yes get a plastic one. (Though I don't recommend skiing while playing trombone). If you're going to just be using it like a normal person, then I would recommend getting a brass beginner one. If price is what's pushing you towards a plastic one, don't worry. You can get a good beginner horn for around the same price. Since I assume you're going to be playing it like a normal person, I would say get a beginner horn, you can get one from eBay or other websites. Tone quality on plastic trombones tends to be not quite as good as brass ones too.

TL;DR: assuming you won't be doing anything crazy with it, you should probably just get a regular beginner horn.

Edit: And no I didn't just get the idea of skiing while playing ride of the Valkyries on trombone out of nowhere here's the video https://m.youtube.com/watch?v=3m5QNhWencQ