There are benefits to showing the downloads both as a spread and a single page. Which should I take into account- the view-ability on smaller computer screens or the design appeal of showing them as spreads?
You should take both into account, plus a third consideration: who will be viewing them. Several times I've sent clients PDFs in spreads with first and last being single pages as they would always be in a printed piece, only to have them worry that the page spreads were "too small compared to the first page." After a bit of explanation, they sort of get it, but still aren't sure.
If the idea is that people will download and print the pages, then you have to export as single pages. Very few people can print spreads satisfactorily; even fewer know how, even if their equipment is capable of it.