The quick answer: whatever you use is fine.
I've used both Visio and Omnigraffle for wireframes, sitemaps and flowcharts, so I can confidently state that it really doesn't matter what product you use, as long as you use it well. From my experience, both do the same thing reasonably well, and follow similar constructs.
That said, what you use to create your user experience documentation depends on two factors:
- Mac or PC?
Mac or PC?
Omnigraffle is only for Macs, so if you are a PC user, you are clearly not going to choose Omnigraffle. You are going to use Visio or something else. If you are on a Mac, you could use Omnigraffle, Visio or something else, so you automatically have one more option than the rest of the world.
A nice copy of Visio is expensive. Here's Microsoft's suggested retail price (as of today):
- Visio Premium 2010: $999.99
- Visio Professional 2010: $559.99
- Visio Standard 2010: $249.99
Their Standard edition seems affordable, but to me, it feels a tad limited for the price tag. If you are just drawing things, you have plenty of options ranging from free Google Drawings to a low-cost sketching tool like Balsamiq.
So, my general belief is: you use Visio because you are on a PC and your company is paying for it. Go you! That's the right choice for you. Enjoy it! Visio is nice.
Now, if you are on a Mac, Omnigraffle is a powerful tool that is affordable. Here's The Omni Group's suggested retail price (as of today):
- Omnigraffle Professional: $199.99
- Omnigraffle Standard: $99.99
In contrast to Visio's "cheapest" version, the Standard edition of Omnigraffle could be more than enough for the average UX person, but it's not too much of a stretch to just get the Pro version.
However, the cost of entry for Omnigraffle is very clearly the Mac. Want a laptop? They start at $999. If you were starting totally new, you could certainly buy a non-Mac laptop and have budget to spare for one of the more expensive versions of Visio.
So what's with the big debate?
If you were to Google the question "which diagramming tool is better?" I'm sure you'd feel like there's a disproportionate amount of folks on Omnigraffle, and they are in love with it (which is appropriate, it's a fine tool). I suspect that this is because they are Mac users, and certain Mac users vocally love Mac things, especially when the thing is exclusive to Macs. Also, UX folks who come from design backgrounds may already be on a Macs, hence they already have limited exposure to Visio. And, to be honest, I'd like to see product usage broken down by permanent employee and independent freelancer for each tool. I'm inclined to think that most freelancers are not on Visio, for reasons listed above, while many folks who are working at larger organizations may be on Microsoft products. But that's beside the point. Whatever tool you use - even paper - is perfectly fine.