12.9" v 9.7"
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Pros and cons for the 12.9"
Cons...
1. Bigger, less convenient to carry around/hold.
2. More expensive than 9.7"
3. Display isn't as bright as 9.7"
4. Cannot shoot 4k video.
5. No True Tone display.
Pros...
1. Bigger screen for media and doing work.
2. Faster CPU and GPU.
3. Double the RAM (and yes there is a noticeable difference.)
4. Louder speakers.
5. USB 3.0 Lightning connector speed.
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Pros and Cons for the 9.7"
Cons...
1. Smaller display area to work or watch media with.
2. Still friggin expensive despite being cheaper than 12.9"
3. Less loud/full speakers (still good, but not AS GOOD as the 12.9")
4. Half the RAM of the 12.9".
5. Slower CPU and GPU.
6. USB 2.0 Lightning connector speed.
Pros...
1. More portability, lighter, and can carry in one hand for longer times.
2. Brighter Display.
3. Shoots 4k video.
4. True Tone Display.
5. Comes in Rose Gold.
Both have almost equal postives and negatives... but also things that make up for them.
In the end, the questions to ask are,
1. What is my budget?
2. Would I benefit work and play-wise from bigger or smaller screen?
3. Will I take photos with a full size tablet... if so then 9.7" is the way to go. (If you want to be "that guy/gal.")

4. How many apps will you use at one time, more RAM will make this a better experience... so if you like tons of apps and such open, then the 12.9" is the better choice.
The most important question: What iPad looks or sounds more appealing? My suggestion is to go check one out in-store at Best Buy, Apple or Staples... ask for some professional suggestions and also some personal suggestions... then make an informed decision. Also realize the 14-day return policy is there for not just damaged or defective items, but to make sure you are satisfied with your purchase. I honestly think you can't go wrong with either one. (But I personally think the 12.9" is amazingly amazing.)
Kallum.