If it's mostly short commutes, that's not great for a turbo long-term and those are expensive to repair when they crap out. And being stuck in traffic routinely isn't going to do a clutch any favors if equipped with a manual transmission (does GTI even have an automatic as an option?). I assume the GTI requires premium gas, too, which is a big negative around here in the midwest where 91-93 octane premium costs ~30% more per gallon the regular 87 octane that the Mazda will run. I don't know what gas prices are like in Seattle, though. Could be very different than around here.
As far as practicality.... Eh, the GTI might have an edge over the 3, but it's not by a lot. I like the styling and economy of the 3 better, but obviously the performance of the GTI wins in a race. I just dislike VW's in general, but that's just my own preference, obviously.
It's not really for us to decide here. You have to decide which factors are most important to you and base your choice on that. Objectively, though, I think the Mazda is a better/smarter fit given your use case you've presented unless you don't care about costs of operation and maintenance.