"I've been interested in your company. Its culture and the opportunities are a great match for me. I'm excited about finding the best role for me there." Or something like that, in your own words.
People want to see that you're interested in the company, and helping them be a success, not just that you're interested in a particular role.
It's honest - you've stated your desire to be at that company - and this will give you an edge over people who just choose jobs, not employers.
Also, it'd be best to not say anything negative about the other role, only what's positive about this one.