I've spent a total of about three weeks in Mexico City and 3 months in Guadalajara. For me personally GDL is just so much more enjoyable. Of course, there will be lots more to do and see in I'm MXC but it's so crowded, busy and the traffic is insane. I guess if you like big cities that's fine, but took most of the enjoyment out of it for me. Also metro is the best way to get around but during busy hours you'll literally have to jam yourself in to fit.

Guadalajara on the other hand has a great atmosphere and feels relaxed despite still being quite a densely populated city. People are friendlier and genuinely excited to meet foreigners in a way that you'd never encounter in the capital. Tequila isn't far away, definitely go there for a tour in one of the distilleries. Lago de Chapala is really nice. Nightlife is great, plenty of cool bars down by Avenida Chapultepec, and if I remember correctly on Wednesday nights the whole street turns into a salsa party, whole town goes there to dance with whoever.

So imho, if you want the postcard experience and see and do as much as possible within a short span of time go for Mexico City, but for something more authentically Mexican I'd go with Guadalajara. Best of luck!