AsmTK > AsmJit
AsmTK, as the name implies, is an assembler toolkit that uses AsmJit as a base for everything and builds additional features on top of it. The currently only feature it offers is AsmParser, which implements a parser that can consume assembler string and emit it into CodeEmitter. More features like Linker are planned in the future.
AsmTK is a first library that takes advantage of AsmJit's instruction validation framework, which requires just 5kB of data to validate everything AsmJit understands. This feature was necessary to build such tools and it's now an integral part of AsmJit, which can be enabled per CodeHolder and/or Assembler.
x64dbg > AsmTK
It didn't take long for AsmTK to gain some attention. The first project that officially started using AsmTK is x64dbg, an open-source debugger for Windows. It uses AsmParser to encode asm instruction(s) on-the-fly and it currently allows to select between 3 assembler engines to do the job. It will be interesting to see which engine people prefer the most, and which will be the most reliable one to use.
The collaboration started after the author of x64dbg pointed out that all of his ~150 tests were failing when using AsmTK as a backend. I have incrementally fixed most of them and integrated a better test suite into AsmTK itself, so we can add more tests and have them run on continuous integration servers. Some tests were failing because AsmTK was parsing instructions in a case-sensitive manner (so all UPPERCASED instructions were failing without any reason), however, other failures required a lot of fixes in AsmTK and AsmJit themselves - for example REP and REPE/REPNE prefixes were completely reworked (I have planned that, but these failures accelerated my motivation to fix that now). Other minor reorganizations in AsmJit happened mostly to increase compatibility with other assemblers.
AsmTK needs more people using it and reporting bugs. I'm completely open to implement many features and to accept pull requests that extend its functionality. AsmTK is much more open to extra functionality than AsmJit in this regard, which I try to keep lean and mean and focused on JIT (while providing a foundation to build on top of it).
So, if you need to parse some asm, take look at AsmTK and join AsmJit's room on gitter. You will get pretty powerful functionality in just 250kB (and the size can be reduced further by disabling some AsmJit features such as Compiler).