Whenever I need to replace a alternator I always upgrade to a 150amp.
To answer your question, it all depends if you are money tight, then buy the used alt, or if you have the money for a new one, then buy new over used. Whenever you get a used item, no telling if it will last a week a day or a year, it is a gamble everytime.
2015 Race Red Performance Package GT 5.0 (435hp) stock
Still in the making Updates coming soon!