The shifters are a big deal, IMHO.

The newer Soras have the dual-cog feature as well, which I like a lot; if you're coming from the small to the large chainring you can double-shift the rear cog at the same time (down two on the cog, up on the chainring) and with most gearing setups this puts you in a logical place (slightly higher gearing than you had before.)

With the upshift button on the top in the older design you can't shift up reasonably while on the drops whether on the bottom or on the curve. That kinda sucks as the only place you really have bidirectional shifting is when on the hoods.

That change is enough of a reason for me to want the newer design (which is very close to the 105 group in terms of functionality) and for older builds would drive me to the 105 group.