Turbo Basic, OSS BASIC A+/XL/XE variants, as far as vintage BASIC's from the 80's go, are all 100% backward compatible with Atari Basic. In fact, the three OSS variants are Atari Basic improved and improved again, as OSS was the company that wrote the original BASIC Rev. A (Atari programmers made B and C revisions). Atari chose not to keep using OSS's latest revisions so OSS sold them on their own. But, all these BASIC's have extended commands so if you use any of them, the program won't run on Atari BASIC.
Modern BASIC's including Altirra and FASTbasic, from what I understand, are also backward compatible with Atari BASIC. Every alternate version mentioned is also slightly faster than the one before the latest ones at least 10 times as fast as Atari Basic.
I use OSS's BASIC XE myself, because it is compatible with SDX and you can move freely back and forth between them, while with Turbo BASIC, it can be launched from SDX, but you have to save your work from inside BASIC before you return to SDX. I can also use SDX's 65 and 80 column software text modes with BASIC XE, which makes it so much better than 40 columns of TB. Though I think you can use TB with other 80 column software drivers and custom roms like Omniview with 80 column support. Plus with OSS's BASIC XE I get very simple to use extra memory, upto 128K for BASIC XE and also along with SDX using extra banks for it and ramdisks on top, I can actually put to use all my 576K or 1088K on my two Atari's to full use while in the BASIC XE/SDX environments.
Though IIRC, Turbo BASIC is faster than BASIC XE, especially if compiled. Probably about 10-15% faster, but BASIC XE already runs ATARI BASIC programs 5-10 times faster! And of course Altirra and FastBASIC are even faster,marginally.
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Not old. Vintage. :)