It depends on what you want to stream, if you have Amazon Prime then a Firestick is the best, the different types you refer to are described
here but for an extra £10 the 4K firestick is faster and picks up weaker wifi signals better although in the van that shouldn't be an issue, I have a Roku stick and as well but I find the 4K firestick is perfect for my needs but I use it on a large 4K TV as well as when we are in the van, you wouldn't notice the better picture quality on a smaller TV
You can watch Britbox on a firestick but not on a Roku stick but most other apps work on a Roku which is cheaper at
£30 for the HD stick and £40 for the 4K