The only difference between plain IFN-α 2a and IFN-α 2b is in the amino acid present at position 23 of the protein: IFN-α 2a has a lysine at that position, whereas IFN-α

2b has an arginine.

However, if you're talking about Pegylated IFN-a 2a and 2b they are completely different.

PEGIFN-α 2b consists of the binding of IFN-α 2b with a linear PEG chain, forming a 12-kDa molecule.  It is more rapidly absorbed (with an absorption half-life of 4.6 hours), has a wide volume of body distribution (approximately 0.99 L/kg) and a mean elimination time of 40 hours.

PEG-IFN-α 2a is formed by the binding of two 20-kDa chains with IFN-α 2a, resulting in a complex 40-kDa molecule.  It is absorbed more slowly (absorption half-life 50 hours),  its distribution is restricted to well-vascularized organs with good perfusion, such as the liver, and it remains detectable in the serum for one week.