I've finally broken ground for my horizontal slinky system. My installer received the Tranquility 20 rather than the 27. Before I move forward with the install of the 20, can somebody clarify the difference between the two in both efficiency and rough $ difference. I don't necessarily want to go through an exchange, but if there is a big enough difference I may consider it.

A 20 is a single stage unit. The 27 is a two stage unit. If you are in a cooling dominated area with high humidity, you definitely want the two stage unit as it will give you longer run times and better humidity control. The two stage unit probably costs $500- $800 more to purchase. Two stage can be more efficient, but the big advantage is comfort dealing with humidity.

Well, I'm in the Poconos, where I would classify it as a heating dominated location, with just 3 solid months of cooling and maybe 2 months of sporadic cooling. Humidity isn't generally a major issue other than July and August. At a $800 premium though, I'm not sure why I wouldn't just want to go with the 27 unless it's just not that big of a difference.

I would probably go with the two stage unit.

I would second the two stage, for both comfort and efficiency.

Two stage - less blower noise, as well - most of the time the unit will be on low stage and the blower moving less air.

You'll experience more even heating and cooling in moderate weather - longer run times mean gentler temperature swings.

The TT20 is definately the inferior choice. If you are okay with going single stage to save installation cost, go with the GS model and get R-22. Heat (gain) load is the best way to determine if the 2 stage will greatly enhance dehumidification, but I agree that the TTV027 is quieter and extremely even in delivery, but savings will be nominal.
Good Luck,
J