I had a very tight T5-T5 connection about a year ago (not planned; weather problems during the January 2013 storms) and was told at the gate that I needn't have worried: as soon as I went through the final security check, the gate staff knew where I was and would have held the flight for a minute or two if necessary.

That said, I'd take the T3-T5 connection here: an hour is more than the terminal transfer will take, so it effectively gives you at least 30 minutes more time.