It is important to make a distinguish between the programming language, the framework, development methodology and the people in your team.
I would say language is least important. Basically the same end result can be archived in any language be it Ruby, PHP, Java or C#. Trying to ask people to pick will only start a flame war, consider them all equal. They can also all scale, in terms of handling volumes of traffic, equally well.
What is more important than language choice is the gains that can be made from selecting a good framework for any given language. For example a popular framework methodology is MVC (Model View Controller).
Some MVC frameworks include RubyOnRails, CakePHP, Struts (Java), and I believe Microsoft have just released a MVC framework for C#/.NET.
The long term benefits of using an MVC framework which allows for separation of concerns - business logic, querying of data and presentation/User Interface - is essential. Any web developer worth his/her salt will use an MVC (or similar) framework for their respective language. This choice will help prevent your developer and designer stepping on each other toes.
More important than the framework of choice is the development methodology, you will want to look in to Agile methodology. Agile is a banner for a method of developing an application in incremental steps where you release improvements/features on a regular basis based on feedback. You deliver the features which hold the most value to your end users first.
Linked in closely to this are development best practices, is your development team going to be producing documentation, writing tests, using version control, deploying to a staging area?
But most important are the people you choose to hire. It is the people in your team not the technologies that will ultimately help your succeed or otherwise. I would suggest hiring the best, not the least expensive from the beginning, it will be less expensive in the long run and give you a better chance of success. Work together to develop the features which will deliver the most value to your end users with your starting capital.