The Mithril Shirt is only 'better' than Mithril Chainmail if you have a DEX in excess of 18. At 20 DEX, it's a toss up (AC 19). At DEX 22 it's better.
Then, there's Mithril Breastplates... [/QUOT
The Chain Shirt is the better option for arcane spellcasters or thieves. There's a 10% less chance for spell failure and the chain mail has a -2 more to armor check penalty. If either of those things are important to you, you are better off with the shirt. In addition to that, if either of those things are important to you you probably have and/or have the ability to have a dex above 18.
Mithral chainmail, atleast I think, is worthless. The 20% spell failure is to high and a mere +5 to AC isn't that good.
Just my opinion.