I have been told that the first of the two acids is the stronger acid. According to this person,deprotonation for both occur at the nitrogen.The resulting conjugate bases include,respectively a neutral amine group,and a negatively charged amine group.Because negatively charged nitrogens are highly unstable from an acid/base standpoint (consider lithium diisopropyl amide)... the first must be the stronger acid (because of its more stable conjugate base).
The problem I have with this line of reasoning is that it's based on a faulty premise - that the nitrogen is the site of deprotonation.I see a much better site of deprotonation for both - and that is the carboxylic acid site. A negative charge here will not only be resonance stabilized but also inductively stabilized by oxygen.Plus, the Ka of ammonium ion is 5.6*10^-10, while the Ka of acetic acid is 1.8*10^-5.In other words - orders of magnitude of difference. I can't see why something that is structurally analogous to ammonium ion (much less ammonia) would be deprotonated preferentially over a carboxylic acid.