Your architecture choices change the way you write your application code and how you utilize the Rails framework. On the other hand HAML is not going to change the way you write markup, it just provides a different syntax that some people prefer. Postgres has features people like that mySQL lacks. MiniTest & Rspec are different syntaxes for testing. These three choices have very little to do with how you use Rails & write your app code, they just happen to be made by the same people who advocate big architecture changes.
I fall in a camp where I use HAML, postgres & Rspec but I also use fat models and skinny controllers. I have a few really small services in my apps but I try to reserve them for cases when the standard Rails pattern is extremely ugly rather than actively seeking out replacements for the Rails way.