I have Sora on one roadie and 105 on the other.
The 105 differs from the Sora in that there is a "trim" function on the 105 triple, causing a need for a little extra "shift" while in the middle chain ring on the front der as the rear der goes from the smaller gears to the larger gears and vice versa, in order to keep the drive train more properly aligned, and reduce chain drag.
The 8 speed Sora does not have (nor does it seem to need) this function.
The thumb shifting does not bother me as I spend little or no time in the "drops."
I like the 105, but I think the Sora is plenty adequate.
Additionally, I do find the 105 brakes much more "firm" in their usage, and find the Sora brakes to heve more "give" in them. I don't know if this is common to all Soras, but the braking/pull efficiency on my 105 is noticeably better. I have heard that there is some flexing in the Sora brakes.
However, my 105 is 8 years old now, and the Sora is a 2004, so likely much has changed.