Riak in SasS application
The key architectural difference between the two, is in CAP configuration (as-in Brewer's CAP Theorem).  Riak is considered to be an "AP" database (High-availability, Partition-tolerant), whereas MongoDB is a "CP" (Strong-consistency, Partition-tolerant).  The underlying question becomes whether the more-important aspect of your system is to read your data or to write your data.  If successful writes are more-important, and you can handle the fact that not all nodes will be consistent (or will be via "eventual consistency") then I would say Riak is the better choice.  If the more-important aspect is that each read returns accurate data a high percentage of the time, and you're prepared to sacrifice a little performance (due to locking) to get it, then MongoDB is the better choice.

To get an understanding of databases/tables/buckets, make sure to read-up on the Riak data model before getting too deep in your application's design.  The only "gotchya" I would caution against, is that I've heard people use many words to describe doing a Map-Reduce, and "easy" was never one of them.  If that functionality is a key component of your application, I'd make sure to investigate that thoroughly before making any decisions.

Here's an article that does a comparison of Riak vs. MongoDB.  It's by Basho (full-disclosure), so it may have some bias.  Additionally, here's a comparison of several NoSQL databases that you may find useful as well.