As an expertise i have experience in advising startups and entrepreneurs on their digital product strategies. When it comes to building an MVP (Minimum Viable Product) for a new business concept, the choice between a mobile app and a responsive website depends on several factors, including your target audience, functionality requirements, development costs, and testing goals.

*Pros of building an App as an MVP:*

1. *Native experience*: A mobile app provides a native, seamless experience for users, leveraging device features like camera, GPS, and notifications.
2. *Offline access*: Apps can function offline or with a slow internet connection, making them ideal for users who need access to your service on-the-go.
3. *Push notifications*: Apps allow for push notifications, enabling you to engage with users and drive retention.
4. *Security*: Apps are generally more secure than websites, as they have better control over data storage and encryption.
5. *Monetization*: In-app purchases and subscriptions are well-established monetization models.

*Cons of building an App as an MVP:*

1. *Development costs*: Building a high-quality app requires significant investment in development, design, and testing.
2. *App store approvals*: Your app must meet app store guidelines and pass review processes, which can be time-consuming.
3. *Limited reach*: Your app will only be available to users with compatible devices and operating systems.
4. *Updates and maintenance*: App updates require re-submission to app stores, and maintenance can be complex.

*Pros of building a Responsive Website as an MVP:*

1. *Lower development costs*: Building a responsive website is generally faster and more affordable than developing an app.
2. *Wider reach*: A website can be accessed by anyone with an internet connection, regardless of device or operating system.
3. *Easier updates and maintenance*: Website updates are simpler and more flexible than app updates.
4. *SEO benefits*: Websites are more discoverable through search engines, driving organic traffic.
5. *Cross-platform compatibility*: Responsive design ensures a consistent user experience across devices.

*Cons of building a Responsive Website as an MVP:*

1. *Limited native functionality*: Websites may not have access to device features like camera or GPS.
2. *No offline access*: Websites require a stable internet connection to function.
3. *Limited engagement*: Websites may not offer the same level of engagement as apps, with limited notification capabilities.

Considering your target market of millennial smartphone users and your long-term vision for an app, I recommend starting with a responsive website as your MVP. This approach allows you to:

1. Test your business concept and validate assumptions with a lower development cost.
2. Gather user feedback and iterate on your website, refining your service before investing in an app.
3. Establish a web presence and drive organic traffic through SEO.
4. Develop a user base and build a waiting list for your future app launch.

Once you've validated your concept and refined your service, you can invest in building a high-quality app, leveraging the knowledge and user feedback gathered from your website MVP.