"They are both under contract, but both are undoubtedly available to us and I think they are both are very good drivers."
He said it was a "very close decision" to sign Perez over Scot di Resta and German Hulkenberg, who are both still linked with Felipe Massa's Ferrari seat.
"I think what we have with Sergio is someone who is perhaps less developed but has masses of talent, who doesn't make mistakes and is a very good racer," Whitmarsh explained.
"We were looking at potential and I think what we were trying to do was pick someone who was going to progress beyond where they are now. We think he will, but we don't know."