There is no such thing as "the x86 ABI". You're probably referring to the GCC ABI or something. For x86-64 there is the AMD64/System V ABI but, again, it's not "the x86-64 ABI". Linux uses these, I don't know about other OSes. For ARM there are a few different ones in use, namely soft and hard float ABIs.
Personally I would say x86-64 is easier than x86 because of extra registers. The System V ABI is a bit more complicated because arguments are passed in registers, which is great if they all fit, but makes it more complicated if they don't (it does encourage you to try to make them fit, though). The ARM ABIs are very similar in this respect, though.
Personally I would say x86-64 is easier than x86 because of extra registers. The System V ABI is a bit more complicated because arguments are passed in registers, which is great if they all fit, but makes it more complicated if they don't (it does encourage you to try to make them fit, though). The ARM ABIs are very similar in this respect, though.