I have personally chosen to purchase two grinders. I have an electric mill and a hand mill for use without electricity. Some mills can be used both with or without electricity. Some hand mills can be motorized - though I do not personally have the skills to do this. I decided to buy one for both categories as I have based a lot of my plans for storage on the possibility of not having electricity in an emergency.
I really like my K-Tec/Blendtec grinder (also called Kitchen Mill). It is very loud, but is compact and stores easily in my cupboard. I can make wheat that is very coarse (cracked) to very fine. My main requirement for choosing this mill was that it makes a very fine flour. The Kitchen Mill also didn't cost too much (about $150). I have not used my Back-To-Basics hand mill (cost $60). My neighbor purchased the Back-To-Basics mill and it broke after not very many uses.
I purchased my K-Tec/Blendtec Kitchen Mill several years ago. Grinders have been updated and revised since then. Because the purchase of a grinder is very personal, I would recommend reading many different reviews before purchasing one. There are several characteristics that differ with each mill. You'll need to decide how important these characteristics are to you. You'll also have to decide how much you are willing to spend.
Things to consider:
How loud is the grinder? (this didn't matter to me)
Can the grinder do a coarse mill, liked cracked wheat?
Can the grinder mill to a very fine flour which is fine enough for your cooking habits?
How long does it take to mill a cup of flour?
How much physical exertion is required?
What is the temperature of the resulting flour?
Do you have to mill multiple times to get the consistency that you want?
Can you also grind corn, oats, rice and/or beans?
Is the mill chamber self-cleaning?
Does it store well in your cupboard?
Does the grinder have to be mounted?
Can it be motorized or does it have a manual setting?
Is it durable?
Does it have a warranty?
Does it use stones or burrs? (There is some controversy about grit and aluminum in the stones)
How much wheat/flour do the grinder containers hold? (In my opinion, this doesn't really matter)
How much does it cost?
How much will you use it?
Because I haven't personally used most of these grinders or tested them myself, I highly recommend perusing the following sites which review, demonstrate and/or compare many of the grinders. The Walton Feed grinder site is superior for reviews of all the manual grinders and a few electric mills. Some stores, like Emergency Essentials, will let you go in and use the grinders and feel the fineness/coarseness of the flours. You might also ask your neighbors if you can try their grinder to get a feel for which grinder you like best.
Great Grinder Information:
Walton Feed Grain Grinder Comparison Pages
The Church of Jesus Christ of Latter-day Saints
Safely Gathered In - demonstrates the Whisper Mill (now called Wonder Mill)
Everyday Food Storage - Good overview of pros/cons and pricing. Does have a bias towards the Wonder Mill, which they sell.
You Tube - Videos of many different mills being used.
Choosing a Grain Mill
Casaubon's Book - Grain Mills
See the comment section for some additional information about grinder "noise" and warranties.