The "Export Details" to Excel option will produce only the detail rows. With only the detail rows available, the exported file is raw data that is easily modifiable. This also allows for a smoother import. The fields used as grouping for summary format reports will also be placed as the last column in excel once the report is exported via Export details hence any report will be rendered as tabular report.
Note: Because Custom Summary Formula fields are a part of the Summary rows, these would not be included in the Export.
The "Printable View" of a report retains the format and summary information of a report. This includes Custom Summary Formula fields.
Note: Because the intention of the "Printable View" is to produce an export which represents what is shown on the screen, opening it in Excel, might show some data as text instead of numbers. Data shown as text, can not be used in Excel formula's as numbers. If you want to use the data, we advise you to use the "Export Details" button.
An example of this would be, when your Salesforce Locale setting has a space character as the thousand separator, upon using "Printable View", Salesforce exports a "Non breaking space", as to make sure numbers are not shown on multiple lines instead of one line. When you have set up Excel to use a space as the thousand separator, it will not recognize the "Non breaking space", and you will not be able to sum these.
After opening the file in Excel for Windows, you could do a search and replace for a "Non breaking space" by entering Left-ALT 0160 (on the numeric keypad), replacing it with a normal space.