Answer depends on business requirement though i will suggest to keep Query Parser “AND”. It will narrow down the search result rather than expend search response.
eg. If a user search for string “Mohit Bansal” means that he/she is looking for results having both Mohit & Bansal, If we search with OR, then it will result with all document having Mohit or Bansal or Both.
My Suggestion: Update schema.xml for solrQueryParser – AND