First of all...in order to get a good answer for this question you need to supply more information.  Have you made a scalability plan for this site?  what type of application are you planning to run? Do you have a budget?

Both venues have their pros and cons, if you a large quantity of cheap machines you will need more robust load balancing and networking facilities.  If you higher performance machines you may be able to reduce the cost of this infrastructure.  Take into account the type of app that you plan to run and the type of related infrastructure such as DB servers and such. 

In addition, if you settle on a brand name you can create standard images that will last for a while (18-24 months) which means that adding the next server will be a very simplified job as opposed to installing each separate machine with its relevant drivers and different HW configurations.

If you post more information, I will be able to give you more accurate advice.