All the prickly lettuce I've seen (called milk thistle here) resembles dandelion more than sow thistle. So, I have to remain resolute in my ID here...plus the spines appear as the weed matures...they're not there in the early stagesBit late, but my guess would be prickly lettuce as, I believe, the spiny sow thistle does not have that line of spines down the stem.
...
So I say Lactuca for this one.