Knitty or MagKnits would provide you with possibly more exposure than your blog or ebay shop, so more people would start recognizing your name. If either one decides to publish your pattern, you'd get around $75-100 for Knitty or $36-90 for MagKnits. (Plus you get the warm fuzzy feeling in your heart when you know that knitters now have access to your fabulous free pattern!) If you publish it on your own, you have to decide if enough people will buy your pattern to match or beat the amount of money you'd receive from an online publication.
You also might want to browse the Knitty and MagKnits archives to see if something like what you're making has already been published--that might discourage you from going that route, since they're probably less likely to publish something very similar to what they've already done. I found
Satchel on Knitty, which sounds somewhat similar to what you described, but on the other hand yours could be completely different. (I dislike the way the MagKnits archives are set up, so I'll leave the browsing to you.)
When it all comes down to it, if it were me, I'd go for Knitty or MagKnits (or even The AntiCraft, although it looks like they don't pay for submissions) for my first couple of designs in order to get the large readership and the consequent publicity and name recognition. You can always pimp your blog in the little bio they give you, so people who like your work will click over and find more of your designs.
ETA: I have not published any patterns, so you can take all this advice for what it's worth.