I used the Nippondenso alternator and I'm glad I did. The Nippondenso alternator is the one with the sheetmetal rear cover. The voltage regulator. rectifier and brushes are under that cover and easily tested and replaced if needed in the future. The Mitsubishi alternator is not so easily serviced ($). I got my rebuilt alternator from Premium Starters and Alternators, online, for $66 no core charge. For a 3/8" or 10mm V belt pulley, order for a '88 Pontiac Firefly (Canadian Model). The 3/8" V belt pulley also works well with a 1/2" Gates XL Green Stripe Belt. For a three rib flat belt pulley, order for a '89 Geo Metro or newer.
Remember that the Nippondenso and the Mitsubishi units were interchangable on these vehicles. I'm a big fan of the ND.