I struggle choosing. I started both at the same time and keep going back and forth.

Riak

  -> I love the fault tolerance
  -> easier to scale with a dreamworld of hash-rings I wish I could have.
  -> no indexes
  -> no Geo-spatial indexes
  -> I don't like link walking... it's not intuitive.
  -> I want to use Riak (Lucerne/Solr like) full text search, but the docs are poor and have to read dozens of useless documents to get nowhere very quickly. It's a separate install that requires pre-commits?(which are never explained well enough to start using it).
  -> installing on Mac OSX is painful (homebrew works/doesn't work depending on version, 32 bit or 64? how come I don't have include 32bit flags with older repo's, yet newer ones are only 64bit yet don't compile.... errors errors errors...ERLANG which? such a pain in the ass installing Riak unless you do it using specific build tools.
  -> Only Joyent provides a hosted solution which is too expensive relative to its offering.
  -> accepts any docs just set the content type.. handles original documents without conversions
  -> easy to load balance behind NGINX
  -> can choose between conflicting writes 

MongoDB

  -> straight forward install.. up and running in no time
  -> SQL like queries
  -> intuitive indexes
  -> Geo-spatial indexes
  -> Affordable hosted services with more than one provider.
  -> no full text searching over documents
  -> have to fiddle with doc handling GRIDFS etc..
  -> can't choose between conflicting writes  (last one wins)

I wish I could have "RiaMong"

  -> straight forward install.. up and running in no time
  -> fault tolerance
  -> easier to scale with a dreamworld of hash-rings.
  -> indexes
  -> Geo-spatial indexes
  -> Affordable hosted services at with more than one provider.
  -> full text searching for documents
  -> handle original documents without conversions
  -> can choose between conflicting writes 

Oh well maybe someday :)