Answering my own ST-2300 vs. ST-2400 shifter question, I find that ST-2400 was introduced last year as a replacement for ST-2300, and indeed the big difference is the change from button to lever shifter. But whether this is an improvement seem to be a matter of debate.
Originally Posted by DPDISXR4Ti
Gone is the thumb shifter of the 2300 groupset, replaced by the mechanism found on all of Shimano’s upscale mechanical groupsets, in which the brake lever and a second lever operated by the index finger control shifting. Shimano claim the design offers a more intuitive feel; a sentiment Campagnolo is unlikely to share. The gear indicator, superfluous to more experienced riders and notably absent on Shimano’s more sophisticated groupsets, is retained from 2300
Read more at Shimano unveil new Claris groupset | Road Cycling UK