I use Mail on my Mac because it's a native app and I expect it to be the least resource hungry.
I have generally found this to be the case.
However, I use Thunderbird (Betterbird actually) on my Windows computer, and I think it's superior in many ways, at least as far as feature set goes.
I prefer Apple Mail but also actively use Betterbird. If Betterbird has significant features that Apple Mail doesn't, I haven't noticed. Note, I still use Apple Calendar and Notes for all my accounts and I prefer that to the integrated/Outlook-like approach of Thunderbird/Betterbird.
How's Thunderbird on Macs, specifically M-series (mine is a M2 Air).
Any noticeable lags or battery impact vs Mail?
I started actively using both on the same laptop somewhere around Catalina as I found that Apple Mail made it too easy to send e-mail from the wrong account (i.e. sending business contacts e-mail from my personal account or vice-versa). Separate programs make that unlikely.
I also wanted to build some escape hatches from the Apple ecosystem. If I ever leave, I will need a solid mail solution that isn't Outlook, and I'd rather explore alternate solutions over time in day-to-day activities than have to deal with replacing everything all at once.
20 years ago Apple Mail was a solid mail program that did what was needed but didn't seem in any way revolutionary. Yet no one seems to have matched it (at least on the Mac).
Thunderbird/Betterbird is more resource hungry -- showing up with about 4-5x Memory Usage in Activity Monitor. I also see more background CPU usage from Betterbird. My guess is that the difference between the two will be measurable but not dramatic on your M2.
These days, one thing I like about using Thunderbird/Betterbird is that it decouples mail data -- and the application in general -- from the OS. With Apple Mail, trying out a new macOS means trying out a new mail program and Apple immediately does a one-way upgrade of the mail data on first run so you can't go back without redownloading (in the case of IMAP) or restoring from backup (in the case of any archived or local mail folders).
Apple generally left the mail data structure alone after the first few versions back in the MacOS X 10.1, 2, etc days. Between MacOS X 10.1 and 10.14, Apple went through 6 versions of the on-disk structure. Since 10.14 / Mojave, they've tweaked the format with every single version of macOS such that it is incompatible with previous versions and every upgrade involves a one-way conversion.
Any specific Mail features I'd lose switching?
If you are using Betterbird on Windows and it works for you there I doubt you'll notice any loss. However, I generally prefer Apple Mail's search. Searching for text in the body of e-mails in a folder is slow under Betterbird and you can't really do anything until it finishes.
I also notice that Apple Mail tends towards inline attachments while Betterbird seems to prefer attachments as obviously separate files.
Generally I find Apple Mail's composition window prettier but (historically) harder to control precisely. Apple Mail seems to employ behind the scenes formatting codes like Word's that you can't see or edit directly but can get into a complex state that's hard to correct. I am more confident that what I send out with Betterbird will look the way I expect on the recipient's side.
P.S.I am assuming you have no interest in the AI features Apple has been integrating or plans to integrate into the latest versions of Mail.
