Have a physical server from a cost perspective its upfront sunk in cost. Where as cloud is cheaper to begin with. From a startup or mvp perspective its probably cheaper to approach with cloud

Thats right and I am aware of that. I guess my question was to vague, so what i am looking at is the following: 2 different IOS App (A and B) using the same Node.js as a backend on the same instance.

Hmm, i guess this really depends on your current skillset. If you are considered an expert in a niche enterprise software, you could provide some form of advisory or consulting services. Alternatively, i got a couple of mates who write mobile applications as residual income, but to be honest its not a lot of money but it does help to build up their portfolio.