HTTP + REST Vs. AMQP + RPC

• REST, RPC - architecture patterns, AMQP - wire-level and HTTP - application protocol which run on top of TCP/IP
• AMQP is a specific protocol when HTTP - general-purpose protocol, thus, HTTP has damn high overhead comparing to AMQP
• AMQP nature is asynchronous where HTTP nature is synchronous
• Both REST and RPC use data serialization, which format is up to you and it depends of infrastructure. If you are using python everywhere I think you can use python native serialization - pickle which should be faster than JSON or any other formats.
• Both HTTP+REST and AMQP+RPC can run in heterogeneous and/or distributed environment

So if you are choosing what to use: 

HTTP+REST or AMQP+RPC, the answer is really subject of infrastructure complexity and resource usage. Without any specific requirements both solution will work fine, but i would rather make some abstraction to be able switch between them transparently.

If you want to build HA infrastructure with minimal complexity I guess AMQP protocol is what you want.

Advantages of RESTful services are:

• they well-mapped on web interface
• people are familiar with them
• easy to debug (due to general purpose of HTTP)
• easy provide API to third-party services.

Advantages of AMQP-based solution:

• damn fast
• flexible
• easy to maintain
• easy to scale
• cost-effective (in resources usage meaning)

Note, that you can provide RESTful API to third-party services on top of your AMQP-based API while REST is not a protocol but rather paradigm, but should think about it building your AQMP RPC api. We can doin a way to provide API to external third-party services and provide access to API on those part of infrastructure which run on old codebase or where it is not possible to add AMQP support.

If we have high-load project RabbitMQ is damn good piece of software and we can easily add any number of workers which run on different machines. Also it has mirroring and clustering out of the box. And one more thing, RabbitMQ is build on top of Erlang OTP, which is high-reliable,stable platform, it is good not only for marketing but for engineers too. I had an issue with RabbitMQ only once when nginx logs took all disc space on the same partition where RabbitMQ run.