I think it's same meaning. I can understand both but If you writing "pean khong phom" it's better more than pean phom (i think pean phom is brief form of pean khong phom, use in talking but if you writhing pean khong phom sutiable more.) In My Opinion

In this case, it's just like formal and informal sentence. "Khong" represents the formal situation. Thais understand both, but it depends on whom you're talking to.