I did a bunch of research online about tank bags and fretted over fit, volume, etc.
Confused and overwhelmed, I went to Cycle Gear and narrowed it down to two or three that seemed the size/ shape/ features I was looking for. The CG guys let me go out and try them on my bike for fit. It's very helpful to see them on the bike and in the flesh, IMO - especially when comparing different bags.
I could have probably then found it online for $5 cheaper, but the guys in my local Cycle Gear were very helpful and accommodating... and they have to eat, too... so I just bought it there and was done with it.
I don't even know what tank bag I ended up with (a Cortech, if memory serves), but I've been happy enough with it. Unless you're looking for some kind of high-end, specialty bag, they're not all that different, IMO.