Byham has proven he can block and be versatile (play, FB, catch). He's also a draft pick that is signed for 4 years. He's going to get the nod over veterans signed for one year that are without much upside like Paymah and Wilhelm.
Heitman is a good center but I believe he'll have some competition for his job in Baas once he gets back. It'll be interesting as Baas is in his last year of his rookie deal.