What are the differences in tuners?

From the most expensive to the least  (within the Korg brand)...

There is a model of tuner made by Korg called the AT-2 which is based on the tuning scale of Professor Yamane. This tuning scale does not quite match that reported by Professor Seamus MacNeill. IMHO this tuner is a ridiculously expensive, academic toy. It uses an analog meter movement and can break if/when dropped. It retails for about $300.

(Notes:  Korg makes the OM-12T which has many tuning modes and can be set over a wide range.  There is also an older model Korg - I don't know the model number!- which was functionally very similar to the AT-2.  It is not part of Korg's current line.) 

The Korg DT-3 has the ability to "capture" a tuning reference pitch making it very easy to "carry" that pitch to other members of a pipe band. It can also be used to determine the absolute pitch and to balance chanters. Being digital, it can be dropped without much fear of breakage. The "down-side" of this tuner is that it uses LED's and is not always easy to read in the sun while tuning drones. It also will eat batteries if you leave it "on". It can be purchased for about $75.

The Korg CA-10/20/30 can't capture a tuning reference, but has an LCD displayed analog movement which is easy to read anywhere. Being digital, it is relatively indestructible and has an automatic shut-off after several minutes. To "carry" the pitch, you need to remember where it is on the LCD scale, but this is pretty simple stuff. I bought one (retail) for $17.

All tuners have microphone inputs...

My ideal tuner doesn't exist, but would be a combination of the CA-10, DT-3 and AT-2. It could capture a pitch, like the DT-3, and have an LCD analog display, like the CA-10 and have bagpipe-friendly scale divisions like the AT-2. I'd also like to see an extension microphone on a gooseneck and a wrist strap to make it easy to tune drones with only one person.