Those methods should be used for very different purposes.
ArcValue Method (PIData object) reference to a single PIValue object from the server for the associated PIPoint based on the passed time and mode. The input variable mode affects the returned value. For example, if you want to get an interpolated value for this time use rtInterpolated for mode. If you don’t want to get an interpolated value, rtAtOrBefore returns a recorded value at the passed time or if no value exists at that time, it returns the previous recorded value.
Summaries2 Method (IPIData2 interface) retrieves several summary types in a single call over the specified range, and for each interval within the range. It is widely used to get maximum, minimum, averages and other types of summary calculations from a given time range of a specific PI Point.
All in all, ArcValue is used to get a compressed or interpolated value from a PI Point on the passed time and Summaries2 is used to make summaries calculation from a given period of time.
Hope this helps!