There is no difference between the two, they mean the same thing. But "work experience" is the common phrase in North America.

Working experience - would mean that you have routinely used your skills in a work situation, (rather than an academic environment.) Work experience would mean that you have had a specific job in iOS development (either paid or unpaid.) The first feels a little clumsy and I might leave the word "working" out, perhaps clarifying later (if you feel you need to) such as "I have knowledge and experience in iOS development both in an academic and work setting."

"work experience" is a common phrase. I have never heard "working experience" before so I would use the 2nd option.