1) Lower startup cost for 2 part 2) 2 part, couldnt fit my HOB CaRx on my sumpless tank.
3) Both have the ability to "overdose". Your alk pump could freak out with 2 part and kill everything, your CO2 solenoid could freak out and drop your pH in your DT too low. Failsafes are available for both if you have a controller, but the possibility still exists.
4) CaRx is probably going to be more difficult, just due to the number of variables (influent flow rate, effluent flow rate, bubble rate VS just number of mLs of solution added in 2-part). If I had the ability, I would go CaRx. For larger tanks, it's less expensive over the long term, and I'm of the opinion that you get more of the trace elements back by dissolving old coral skeletons than you can with just the chemical additives in 2 part. That being said, 2 part is working just fine for me.