The newer Defy will be no easier on hills and in fact will probably be harder for you based on gearing. Your current has a 11-32 cassette. I don't see specs on 2016 Defy 1 but I'll guess it has a 11-28 cassette. 34/32 is much easier gear than 34/28. Now the 2016 might come with a 11-32 (but I don't see many 105 bikes with them stock) and if not you can change to that cassette (and probably need to swap the rear deraileur to do so). The shifting and hood ergonomics is nicer on the new 105 and discs might be a benefit at your weight but I think it's an incremental increase and considering you'll probably get half of what you paid for your current bike 6 months ago when you sell it seems like a losing proposition

Also, not sure why you can't upgrade the current bike. Who told you that, the guy trying to sell you a new bike?