SuiteScript - Driving data from Invoice to Customer

We're attempting to work with SuiteScript to make some automated processes when we create an invoice.  Basically, we want the start date and end date from our invoice to overwrite the start and end dates on the customer record we are making the invoice for.  When working with this data, I am unsure as to which SuiteScript style to use (ie, the Server SuiteScript, or the Custom Code/Client SuiteScript).  The fields are supposed to be changed when saved.  Currently, the example below is using the Custom Code/Client SuiteScript.

 function customerEndDateFromInvoice() {     // gets the customer being invoiced  var entityName = nlapiGetFieldValue('entity');       // sets variables to hold the data for the start date and end date  var invStartDate = nlapiGetFieldValue('startdate');  var invEndDate = nlapiGetFieldValue('enddate');     // retrieves the customer record that matches the one being invoiced  var record = nlapiLoadRecord('customer',entityName);    //show the information that is being retrieved  alert( record.getFieldValue('companyname') +" "+ record.getFieldValue('leadsource') );  confirm( record.getFieldValue('startdate') + " is going to be replaced by: " + invStartDate);   confirm( record.getFieldValue('enddate') + " is going to be replaced by: " + invEndDate);     // sets on the customer record the start date and