Both kalk and bi-onic can work, but both have their issues. The two part additives are more expensive. On a large tank, or a tank with lots and lots of calcium demand, the cost of dosing bi-onic is significant.
On the other hand, kalk requires a drip dosing mechanism of some kind, must be prepared in advance and frequently, and finally it's limited by the amount of evaporation your tank has. If you check out the old Aquarium Frontier's site, look for a column of Craig Bingman's entitled 'The Limits of Limewater'. It'll give you an idea as to wether kalk on it's own would be sufficient to keep up with calcium demand in your tank.