I assume yo are on Windows since you talk of Outlook. Well, in the last 4 years, I tried and bought most of them. IMO, all the open source one developed on linux primarly look butt ugly on windows(GTK and all simply does not blend well with Windows Native UI) and are a pain to navigate. Plus, they do crash alot on that OS.
I also bought and tried Mailbird Pro. For simple emails and social stuff it's good, the unified inbox work nicely but it uses alot of memory and doesn't support 10% of Thunderbird features that you can get through it's plugin system. They are also insanely slow to add requested feature like PGP or s/mime support.(even though they say they are working on it, they have nothing to show for it after 2 years now...) You can add "app" to it, but not sure if the API is public, I doubt it.
I also acquired EmClient 6 2 years ago: Stable, fast, but outdated security(support SHA-1 certificate only), no support for PGP, no plugin system and always has trouble with Gmail sync every few months(probably due to changes on Google side) that requires an update to fix. Also very slow development. Version 7 should come out in 2016 when it was supposed to arrive in Q1 2015, but it looks like it will still be stuck with SHA-1 and no PGP. I hope I'm at least wrong about SHA.
So, honestly, save money and stay with Thunderbird. It's not perfect, and the interface is dated(especially with no real unified inbox) but it's way more features rich than anything else commercial out there right now.