It really depends on your vision for the app. If this is a single weekend project or a toy web site that you'll likely never touch again, or if you do will only be for minor updates, then I say go for Flask. It'll get in your way less and let you just launch it sooner.
For anything larger than that though, Flask has always turned into a giant headache for me. As soon as I ever start to try and organize the project better because it's become too large for a single file, all shit hits the fan. Supposedly this has gotten slightly better with 0.7's Blueprints, but I've yet to see it. Circular imports and Flask's global variables bite me in the ass on a regular basis. I also have come to really miss tons of features that Django offered, like URLconfs. The decorator technique works great when you can count the number of URLs that your app has on two hands. Beyond that, ack/grepping my project to find where anything lives isn't my idea of maintainable. With Django I can very cleanly trace through the whole request/response cycle in my head and by looking at the code paths.
For anything larger than that though, Flask has always turned into a giant headache for me. As soon as I ever start to try and organize the project better because it's become too large for a single file, all shit hits the fan. Supposedly this has gotten slightly better with 0.7's Blueprints, but I've yet to see it. Circular imports and Flask's global variables bite me in the ass on a regular basis. I also have come to really miss tons of features that Django offered, like URLconfs. The decorator technique works great when you can count the number of URLs that your app has on two hands. Beyond that, ack/grepping my project to find where anything lives isn't my idea of maintainable. With Django I can very cleanly trace through the whole request/response cycle in my head and by looking at the code paths.