Yes, they absolutely make us contenders. It's silly to suggest otherwise. Durant alone makes us a contender. And Horford is really that good. Whiteside is better at the things Whiteside does best, which is blocking shots and crashing the boards. But Whiteside does not play team basketball, and that's a problem. He is prone to getting out of position on defense, trying to go for blocks. He is a complete black hole on offense -- 36 career assist in almost 3400 career minutes. That's an assist once every 3 games for him playing starter minutes. Players don't like playing with selfish players.
That's not to say he isn't talented. He is. But Horford is the better overall player and the better teammate, and he should absolutely be the choice before Whiteside, with or without Durant.