What information is more reliable -- battery capacity or stand-by time?
In my view, battery capacity i.e. the mAh value is best the way to compare.  The reason why I say this is that the battery drain includes so many factors that even a small software glitch can show a remarkable difference.  Hence the standby time is a combination of several factors which are not reliable for comparison, unless they are from from same manufacturer (with a hope that there will not be variations in test situation).  The guide from GSMArena explains this clearly as:


  Manufacturers measure talk time in controlled conditions and the quoted numbers are rarely reached in real-life scenarios. These numbers are best used as reference when comparing phones from the same manufacturer because battery life measurement methodology will probably vary from manufacturer to manufacturer.


With battery rating at least you know your how much capacity the battery has and how it gets used or wasted is simply a temporary situation (i.e. by changing ROM, updates, platform upgrades etc).