My one and only reason to use Mongo over Couch is geo indexes. As far as I can tell this doesn't exist natively. I'm also not sure how Geocouch comes in worth this.
Cloudant will soon offer geo-spatial queries. Its in beta now: https://cloudant.com/product/cloudant-features/geospatial/.
Marketing. They shipped with unacknowledged writes for a long time and it made them look really good in write benchmarks. Couch was actually trying to keep your data safe. But it didn't look fast enough so those that didn't read the fine print on page 195 from the manual where it tells you how to enable safe data storage for MongoDB, jumped on the bandwagon.
Oh and mugs, always the mugs. I have 3 I think.