To be honest, I really prefer the Mini 10 (I now own both Mini sizes). I like the keyboard better--it's larger so you don't feel so cramped typing on it. Plus the keys are all in the right places rather than in odd locations. The slightly larger screen is nice too. You don't have to scroll as much as with the Mini 9.
The function keys are in the middle of the keyboard on the Mini 9 (they share duties with letter keys). On the Mini 10 there is a row of small function keys at the top of the keyboard.
As I said in my post, the Mini 10 is very quiet. You can only hear the hard drive softly whirring away--there is an occasionally light mechanical "chuckle" kind of sound as the hard drive ramps up on boot up. Nothing loud or distracting though--just a quiet and normal laptop sound. The hard drive works faster than the
SSD drive in the Mini 9 which is something I also like about the Mini 10.
The Mini 9 is virtually silent. Some people say they can hear a very quiet hiss from the speakers. I don't hear anything but perhaps some people have extra sensitive hearing.
Battery life is short on the Mini 10--only about 2 hours or so. The Mini 9 can go around 4 hours I think. I am willing to bet that Dell will be coming out with longer life batteries for the Mini 10 soon. It's not that big of a deal to me as I mostly use it on AC.
The ram is limited on the Mini 10--only 1 GB and it can't be upgraded. The new models coming out soon, however, will let you upgrade to at least 2 gb. They will also have more features such as a TV turner or GPS. I am planning on getting a "souped up" Mini 10 later in the summer.
Overall, I do like the Mini 9. It's very small size is perfect for anyone wanting to carry a laptop to a coffee shop, in your purse or pocket, or just to check email on the road. Since I am a teacher the ability to type easier was appealing to me which is why I bought the Mini 10. Also the larger hard drive size and screen size work better for me for both word processing and light photo editing. The Mini 9, while a great little laptop, was just a tad too small for what I needed.
I think that overall you need to identify what tasks you will be using with the Mini--that would help you determine which version might be the best for your needs.