Advantages of a pump per loop:
- More redundancy, making your heating system less dependent on any one pump.
- Finer control. Each loop can be on any zone and have its own speed setting.
- Overhead loops. They almost demand a pump per loop to make sure air is reliably purged from the loops.
Advantages of splitters:
- Lower initial cost.
- More loops per tank. Rather than 2700 square feet max, a tank can serve up to 5400 square feet of floor. In practice it is awkward to run that many loops to one tank. A house that big will usually be served better by two tanks in separate locations.