Question

Just looking for what people see as the main differences when choosing between the two.

Answer 1

Data model

MongoDB

Document oriented, JSON-like. Each document has unique key within a collection. Documents are heterogenous.

Redis

Key-value, values are:

• Lists of strings
• Sets of strings (collections of non-repeating unsorted elements)
• Sorted sets of strings (collections of non-repeating elements ordered by a floating-point number called score)
• Hashes where keys are strings and values are either strings or integers

After Wikipedia.

Storage

MongoDB

Disk, memory-mapped files, index should fit in RAM.

Redis

Typically in-memory.

Querying

MongoDB

By key, by any value in document (indexing possible), Map/Reduce.

Redis

By key.