If you want to play with embedded Linux, get a SBC board with a Cortex A-type processor.
If you want to play around with Linux and general computing, get an old laptop or whatever and install a distro on it.
If you don't want a Linux board, but still need to do fast calculations, get a Cortex m7.
Anything else is like comparing apples and oranges.
Or perhaps apples and pears now that fast MCU's look much like older MPU's.
But still the ARM926 is starting to show it's age, and the IPC aren't that great. The Cortex A9 and even m7 have new pipelines and show quite some improvements in synthetic embedded CPU benchmarks like Coremarks or Dhrystone.
The original RasPi also had a single core ARM926 chip, but now practically all boards now use the Cortex A series. They are really quite significantly faster.
If you ever plan on DIY'ing a Linux SBC perhaps the ARM926 chips are more accessible (sometimes available even in QFP, slower RAM, etc.), but these chips also support much less RAM which may restrict the amount of linux firmware you can run on them.
More over, m7 is probably going to see another big boost this year when newer chips will come out with higher clocks. Atmel has the SAM V series with m7 at 300MHz right now, and I think ST is going to release the STM32H7 series that will run 400MHz.