How to show value to a potential company if they don't currently use related services and you are a start up

I am currently in the proto typing phase of my second venture and have seen lots of positive feedbacks from top VC firms and have an impressive board of advisors but we are struggling to answer the question that is recurring: what value are you bringing to your customer? My partner and I disagree on the model we are operating - he wants to continue on a monthly subscription model with a capped amount of hours for customers, and since I lead sales, I have found it hard to convince companies @ $x/month for x hours they are getting par value or more value than had they used a traditional creative agency. My hypothesis is to solicit any and all projects, and then aggregate that data and six months to a year down the road, see which companies could stand to benefit from the subscription and show them real, tangible evidence that they are benefitting from this. I know it sounds like I have the solution, however, the subscription model is much more sustainable and is a competitive advantage.

Our business model is a tech-enabled creative agency working with the top art schools in the country and servicing companies in the start-up and non-profit sectors.

Feedback would be appreciated.

Thanks