Honestly, for any of the standard rulers, I would think you'd get a better deal at Joann's - either on sale or with a coupon. I doubt you'll find a better deal at any of the vendors.
That said, there will be all kinds of specialty rulers that Joann's doesn't carry (or didn't that I was aware of).
I think the Creative Grids(?) rulers are the ones that are ONLY sold at LQS's. So if you see something they have that isn't at Joann's, then get it.
You don't mention if you're just starting a collection - at which point I'd definitely recommend going the Joann's route, or to just wanting to replace or add to what you already have.
Also, if you have any kind of vision problem, you should check out which one is best for you. For me, it didn't matter.
I have a scores of rulers from all the companies - CG, Omni, Olfa, some of those green ones.
Your money might be better spent on some weird outside-the-box ruler - like a wavy ruler, or a great set of 10 pc nesting circles or ovals (I scored those in Houston one year and love them - I've rarely seen them again).
I also bought the Louisa Smith set at Houston on year - I was never going to see those at Joann's. I picked up the additional sets at a LQS when they came out.
Also, if you need specific custom sizes, there's always Lowe's or Home depot (or you local lucite/glass store).
I'd rather spend the money on rulers like that than something I know I can get 1/2 price at Joann's.
It's not that I never buy a Creative Grids ruler - I'm looking over and am amazed at how many I have. If they come out with a new size that no one else has, then I'd buy it (like the 8 x 24. or the 8x12 or the 20square) - I got those when they were first introduced at a LQS. But now they ARE available at Joann's. So take advantage of it.