spi lib doesnt work with the 84
im using it to send out to shift registers , but I found it easier to just bitbang pins manually (not digitalWrite)
Maybe it could be usefull for you to make wire library working on tiny.
hm I am going to say memory because there is always the option to use an external clock and if one uses a oscillator package its only eating up one pin, adding memory not so easy ...
course in a perfect world, both!
Would digital*Fast functions be useful to you?
including the tinys