If you can work on a green-field project, you'll get a LOT more experience implementing new things that exercise your architect muscles, and you'll be around architects that you can ask why they are making decisions they are. Versus working on an existing project, where you tend to just fix bugs or figure out how to hack in a new feature without breaking what's there. (I don't know which of these camps you're already in, but in general I think it makes a big difference.) (Disclaimer: I am a jr dev, but see a world of difference between what I get to learn and others at my level because of my project.)