I have to admit I am a little biased towards water to water heat pumps. You have so much more flexibility in designing a system with a water to water heat pump than with forced air heat pumps. One thing to consider: when you are producing chilled water with a water to water, you will get hot water. What you do with it makes a difference in how efficient the system is. When you use both the hot and chilled water produced by the heat pump, you effectively double the system efficiency.
An obvious example of how hot and chilled water can be used simultaneously can be found in most hotels and multi-family residential buildings. When the building is being cooled, heat taken from the building can be rejected directly into the domestic hot water tanks rather than into the GHX. If the facility has a swimming pool, it can be heated with waste heat that would normally be rejected into a pool.
Taking advantage of energy storage allows a designer to take greater advantage of opportunities for simultaneous heating and cooling (see the article about GCHP and energy storage). Using energy storage to make chilled water or ice while simultaneously producing hot water in storage tanks helps a system meet the demand for domestic hot water for morning showers, while producing chilled water for the afternoon air conditioning loads.
Buildings with large internal gains, such as retail stores, can benefit by using waste energy to melt snow in the loading docks or sidewalks in front of the building. Larger projects often present many opportunities for simultaneous heating and cooling. Purchasing 1 kWh of electricity and receiving 3 kWh of cooling plus 4 kWh of heating provides an effective coefficient of performance of 7!
In my blog I'll be expressing my opinions about what I've the learned about ground coupled heat pump (GCHP) systems over the last 30 years. I've been very fortunate to work with many interesting people who are passionate about this technology...engineers, geologists, mechanical contractors, drillers, excavation contractors...in different parts of the world. I've learned a lot from them and will be using this forum to pass on some of the things I've learned and feel are important. Please feel free to use this information if you feel it's worthwhile...hopefully you can avoid some of the same mistakes I've learned from.