Here's a nice summary of the pros and cons of each with the requisite Randy Holmes-Farley references:
http://glassbox-design.com/2009/two-...ch-is-for-you/
Essentially, calcium reactors usually cost more to set up and there is a steeper learning curve. Two-part dosing is cheaper with bulk chemicals, but the cost can add up if you're using programmable dosers and such and if you have a large tank requiring a lot of additive. You must also keep up with water changes using two-part because your salinity will begin to creep up over time without them due to ion imbalances.
If you are just starting out, simply going old school and dripping kalkwasser could be the best method until your tank demands outstrip kalk's capabilities.