Asking for a friend, really!

The most important thing is that it's fresh, whichever variety you choose. Spices lose their potency pretty quickly, so if you've had a jar on the shelf for a year, toss it and get fresh.

The best thing is if you can smell/taste the differences at an actual spice shop. I think Sean R. pretty well nailed it. I'd add that, traditionally, basic grocery store cinnamon was Indonesian - but think most people agree that once you try Chinese or Saigon, there's no comparison.

I do think some of it depends on how you typically use it. If in baking, sure, go full throttle on the "caterwauling solos" (nice turn of phrase!). But if it's to complement other flavors (chutney, jam, curry, meat rubs, etc.) you might want something more akin to Ceylon or even Indonesian - so that the cinnamonness doesn't dominate and overpower.

Just adding my two cents -- I never, ever bake or cook with anything but Vietnamese now....once you get used that that stronger kick of flavor it's hard to go back. It's especially great for any dish where you taste cinnamon a lot, like oatmeal or pie or even just sprinkling over apples.

That being said, I work for King Arthur Flour and we just starting sourcing Madagascar cinnamon which I've kind of fallen in love with! It's much more delicate and floral and sophisticated in flavor; it works really nicely with other spices so for something with a jumble of spices, or a more delicate dessert, I'd recommend giving it a shot. Other than those two types, I wouldn't really ever use just plain grocery story cinnamon.

I buy jars of cinnamon at Cost Plus World Markets or Trader Joes. Never really gave it much thought as I only use a sprinkle in my coffee. Not really a fan of using it in many dishes or baked goods....well maybe oatmeal raisin cookies.

I have heard that the Vietnamese cinnamon was the best and also hard to get at one time. What is your opinion Sean? For everyday use (like in my oatmeal this morning) what would you recommend?

Former spice store employee here! If you want a strong presence of cinnamon, use a Cassia variety. If you want something milder or favor using cinnamon in its raw form, consider Ceylon. Always, I tried to dispel the notions of "true" and "fake" cinnamons. Like Halfpint said, one isn't better than another. The naming is just historical/botanical baggage, but that's a story for another time.

Where I worked, we offered:
Vietnamese (a.k.a., Saigon): Cassia, strong, pure cinnamon flavor a la Cinnabon. High cinnamon oil content.
Chinese: Cassia, strong, varied floral/fruity/spicy aromas. I use this in sweet and savory dishes.
Indonesian (a.k.a., Korintje): Cassia, a nice midpoint, fruity
Ceylon (a.k.a., Sri Lankan): Verum (translated as "true"), mild, smooth, more of a team player.

I always order from the Spice House. Same suppliers as Penzeys, but cheaper. Personally, I prefer Vietnamese/Chinese for baking, Chinese for...uh...Chinese dishes, and Ceylon for Mexican. But I still get freebies from my former workplace. :P

My preference is Saigon Cinnamon. I think the flavor is more intense and cinnamon-y than the "regular." I usually buy mine from Vann's Spices on line.

Personally, I like Ceylon just because I prefer my cinnamon singing backup harmonies and not caterwauling solos. But that's just me. HalfPint's answer is great.

I use Penzey's Korintje (sp?) cinnamon because I prefer that fragrance of this variety. It's what most Western Countries know as cinnamon and most recipes that I use are created based on this cinnamon. I've used Saigon Cinnamon which has a "spicier" stronger flavor profile. For me, this changes the flavor of the end product. It's not a bad thing. It's just different.

As for the fake vs real cinnamon, Korintje and Saigon are from the cassia tree. The only true cinnamon is the Ceylon variety and it has a milder flavor more honey and fruit like (not my words, from: http://www.seriouseats.com/2011/01/spice-hunting-cinnamon-canela-ceylon-saigon-cassia.html). The link has some good info about cinnamon vs cassia. I don't think one is necessarily better than the other. They both have their own merits. It all comes down to your preferences.