iMac upgrade - what will I gain?

I have a 27inch iMac, late 2012, with a 2.9ghz i5 and 24gb of RAM. It's running slowly and I’m considering an upgrade. Out of the latest 3.4ghz and 3.5ghz models I’m wondering what actual gains I will see. Can anyone advise on real world gains - I mainly use indesign and photoshop - or what I could do to my existing Mac to make it run quicker. Thanks.