Heh I press reply and in the "Warning - while you were typing...." thing I see many of us said roughly the same thing, I will add to the forum clutter I guess.
It does not matter so much here but in general saying things like "65816/Super Nintendo specific syntax highlighting" is a meaningless term as far as assemblers go. As the SNES is not exactly the most register heavy device (two and an accumulator) it is OK here and things are encoded at instruction level. Still
Some assemblers might interpret "mov r1, r2" as "mov the contents of r1 into r2".
Others though will interpret it as "mov the contents of r2 into r1".
instruction source, destination
instruction destination, source
Both are quite valid to write an assembler with. It can be the source of many bitter arguments, others are made for people that like one way and are coming in from using a chip that does the other way (and then bitter arguments). I would probably still flip a coin and go with that but my gut says "instruction destination, source" is the more common method in general. Again though this is the SNES so you can put that to back of your mind.
Even so you will still find that not all SNES assemblers are made equal (very far from it) but as long as you are not getting too deep into the macros, variables, in assembler maths and maybe some of the pseudoinstructions you should be OK. I am prepared to find out all known viable 65816/Super Nintendo friendly assemblers use one particular method but if you go on to learn assembly language for other chips then remember it is assembler specific (and maybe even version specific) and not device specific.
That said I guess you are probably going to go in for bass or maybe xkas (bass is the better tool but xkas has history and fans among many extensively hacked SNES games) so you have something to work towards. Should you be working with another existing assembly hack of some form then it is up to you to figure it out (and reading around there are all sorts of people using all sorts of things).
On the syntax highlighting thing though why not write your own? If this is a learning exercise it might not be quite as nice as writing an emulator for the chip but it will probably be considerably better than staring at an instruction set for the chip and hoping something sticks. You will still have to disassemble and figure out a whole bunch of existing code and spend your time staring at some combo of ( http://www.smwiki.net/wiki/65c816_instructions
or something along those lines) but it would undoubtedly help in such a process.