If you're referring to the ITX Llama, that'll probably be a better fit than the MiSTer's AO486 core simply because the Llama was purpose-built specifically to do DOS/Windows and to do it well. With the MiSTer you're getting a rapidly growing stack of arcade games and a whole slew of other retro computer systems and consoles included. The ITX Llama won't do SNES, PlayStation or Commodore Amiga to name but a few. If all those other platforms aren't of interest to you at all anyway and you just want to focus on DOS, the Llama looks like a great system to get indeed.
The MiSTer's AO486 core has been subject of a great number of discussions on here already by people questioning how authentic it is. While there is really no such thing as an "authentic" DOS machine, AO486 does have its limits. It's far from an exact replica of an i486 CPU. It behaves mostly like a 486SX to software that doesn't look all that closely at it. Me personally? I'm happy to have AO486 on MiSTer because the alternative would be to have nothing at all, so I'm not complaining. The Llama adds an FPU to the system, which is useful in newer titles that expect a 486DX(2).
For older DOS there's the PCXT core, which is quite close indeed to an IBM 5160 or Tandy 1000 of the early 1980's. The ITX Llama probably won't be as good at running this generation of software because it presents a 32-bit CPU that can have its clock and caches reduced, but it'll always be a 386/486-like system. The XT, being based on a 8088, won't be simulated very well if at all on the Llama. The MiSTer's FPGA has a clear edge there because it does present a very close workalike to a real 8088, CGA/Hercules graphics and other ancient hardware.