Kalkwasser is kalkwasser, there's no doubts about the benefits you'll get outa it. I don't have experience with liquid calcium, but I do know it's a lot pricier to use than kalkwasser
Use kalk, liquid calcium adds only calcium and won't help buffer your alkalinity. Unless you are using a two part additive, like B-Ionic. Liquid calcium is calcium chloride and will leave excess chloride ions in your tank. Long term use can cause problems. However you can use it short term to get your calcium up if that's your goal, then use kalk to maintain it. Going from 250ppm to 400ppm with kalk alone takes a LONG time. After a month of dosing the highest I could get was 285ppm. Just not enough evaporation to add enough kalk to raise it faster. I did it in 4 days with Kent liquid calcium. Now I only use kalk to keep it in that range.
Ditto what psiico said. The limiting factor (and major disadvantage of kalk) is the evaporation rate, but for majority of people, this is not the limiting factor due to relatively high evap. rate and low calcium demand.