See it depends upon your preferences. If you want to caters region specific services then it would be appropriated to get different website. It help you;
1. Focus SEO strategies for different countries.
2. You will have separate performance from different region
3. You can cover greater region for that country
But if your budget is tight then you can consider subdomains. But personally i feel a big website would be difficult to organize and maintain. Also if you want to SEO that big a website, it will be difficult too.
Maybe folders can be the best solution. That's because multiple domains tend to have duplicate content, which degrades the general site in SEO rankings. Also, you will have to brand each of the domains, concerning about each of the audiences.