Hello Luna, I say either. Fifty fifty. Until the 1920 they were used fifty fifty by the population. In the 1940s rely ON became more popular in both the USA and the UK, and in Australia, NZ and Canada. This is because UPON has become less common. However, I like UPON, it has a nice ring to it, and I shall start using it more now! I hope I can rely UPON you to support UPON !

The 1920s ! I MUST wear my glasses when I type!

I guess both are the same. giving the same meaning. some people say that upon might be more formal, while others say it only depends on personal variations and both are okay !

Luna- "upon" is a little formal. I suggest you always use "rely on"