Weight saving between Sora and 105
Since nobody so far has posted weights, I'll leave "one" data in case it's useful for somebody.

                   | 105 5800 | Sora 3500 |
-------------------+----------+-----------+
Levers             |     486g |      496g |
Brakes             |     388g |      357g |
Front Derailleur   |      89g |      105g |
Rear Derailleur    |     234g |      246g |
Crankset           |          |      844g |
Crankset (with BB) |     808g |           |
Cassettes          |     269g |      343g |
Chain              |     257g |      270g |
-------------------+----------+-----------+
Total              |    2531g |     2661g | 



105 is average weights with BB, and Sora is one actual weights without BB.
The sources of 105 are multiple shops (Not in English). I think they just list what Shimano claims but doesn't make it available online for whatever reasons.
The source on Sora is this blog (not in English too, but with photos of each part on a platform scale).
Actual weights will vary depending on the crank length, the gear size, the cassette combination, the derailleur capacity, or even quality of individual parts (yours may be slightly lighter or heavier than average).