Neither is another option.

When working on indexing you need to consider the all the ways a table will be queried to come up with the best indexes. In your one example, the recommended index is probably better than the single column index because it is highly unlikely that you will only be returning ProcessID in your query, so you'll require a lookup (key or rid) to retrieve the rest of the columns used in the query. The issue with the recommended index is that there might be other queries that would use the same key values but return columns other than RunNum, so an index without an INCLUDE might be the best way for the table or an index with and INCLUDE that has more than the just the RowNum column.