Need more info! Need to know whether you want a shoulder strap option; whether you tend to hand carry your bags or slip them onto your forearm more often; what your lifestyle is like.
The three bags you are asking about are actually all fairly similar in ultimate size, though in my opinion the 28 HAC might actually hold the most stuff inside it. Next would be the 32 Kelly, and then the 30 Birkin. But that's just my opinion based on MY stuff, and how I stick it into the bags.