ArcGIS Length and Area Calculation Scenarios

Trying to get a handle on how length and area are calculated in different scenarios in ArcGIS. I don't know why I can't find a answer on the feature class fields, but I can't find a precise answer, unless I'm not understanding something and I know there is some history. Can you help me fill in the question marks? Or tell me why I'm going about this all wrong ; )

GCS = Geographic coordinate system PCS = Projected coordinate system
All links are to 10.1 help docs --

1. 
   

   Feature Class shape_length and shape_area fields
   a. GCS - ?
   b. PCS - Using simple planar
   
   c. Is it always auto-updated, except for shapefiles? yes

   
   


2. 
   

   ArcMap Measurment Tool
   a. GCS - default geodesic, alternatives are Loxodrome and Great Elliptic, but not planar. Area calculation not available!
   b. PCS - default planar, alternatives Geodesic, Loxodrome, and Great Elliptic
   http://resources.arcgis.com/en/help/main/10.1/index.html#//00s500000022000000

   
   


3. 
   

   Attribute Table Calculator
   
   a. GCS - not available
   b. PCS - planar
   http://resources.arcgis.com/en/help/main/10.1/index.html#//005s00000027000000

   
   


4. 
   

   Calculate Field Tool (Data Management toolbox)
   a. GCS - geodesic linear, area available but questionable
   b. PCS - planar
   http://resources.arcgis.com/en/help/main/10.1/index.html#//00170000004m000000

   
   



5. 
   

   Buffering Tool (and other tools coming)
   a. GCS - geodesic
   b. PCS - planar or specify GCS output http://resources.arcgis.com/en/help/main/10.1/index.html#//000800000019000000

   
   


6. 
   

   Javascript API Clientside
   a. GCS - geodesic area and length functions
   b. PCS - can convert from web mercator to geographic (or use geometry service) http://help.arcgis.com/en/webapi/javascript/arcgis/help/jsapi/namespace_geometry.htm

   
   



7. 
   

   Flex API Clientside
   a. GCS - geodesic area and length functions, "The length [or area] will be calculated using a custom cylindrical equal-area projection". This is not mentioned in the javascript api!!
   b. PCS - can convert from web mercator to geographic http://resources.arcgis.com/en/help/flex-api/apiref/com/esri/ags/utils/GeometryUtil.html

   
   


8. 
   

   ArcGIS Server REST API - Geometry Service
   a. GCS - geodesic
   b. PCS - planar
   http://help.arcgis.com/en/webapi/javascript/arcgis/help/jsapi/geometryservice.htm

   
   
   

Another question, what exactly is a geodesic measurement? I thought it meant a 3D trig formula on a spheroid (haversine?). And is it too slow to use in calculating an area and that is why equal area projections are used?

Another question, when determining length and area -- is an equal area projection more accurate than a geodesic calculation using the same datum, spheroid? And briefly why?