Well you didn't say what model 105 or Sora. Those in themselves are just names for a performance level or product tier. Sora is and has been a 9 speed group since introduced I think. Current 105 R7000 or slightly older 105 5800 series are now 11 speed groups on the rear. But at one time long ago 105 was just 5 speed. I think, Let me know if memory is faulty though.
So from my opinion, it's all about gears. Why wouldn't you want eleven if it is a 105 5800 or newer? More choices for hills long and short. More choices for tired legs at end of long ride. Makes a big deal for me. Particularly the hills. It's nice to have gears so close in ratio that you can click once to the next higher ratio gear with out killing your cadence on a climb. Sure you can do it with a 3x5 but you kill your cadence still by having to shift mutiples and you have to know what gears you are in and what you have to go to to find the next. On my 105 5800, the front double is now thought of as a single speed and the action takes place on the back 11. Especially with a 52/36 or 53/39 front.
As far as weight, unless your body is to the point physically where you can't get any more watts out of it, then the little weight difference isn't going to help you. So don't buy for weight alone.
Last edited by Iride01; 04-30-18 at 04:49 PM.