i have space designer, but not altiverb - but i can tell you this. while space designer is really good i can't see it comparing to altiverb in terms of orchestral work and so forth. you should really just check out the demos on audioeases website.
one thing that sets altiverb apart is the fact that you can place instruments in different areas, and it actually changed the reverb. you can somewhat simulate this in space designer by panning and adjusting the wet/dry, but it doesn't compare.
don't get me wrong, space designer seems really nice (and maybe i'd like it better if i understood all the parameters), but from everything i've heard about altiverb, and from all the demos i've heard with it there is just nothing else that compares to it.
I use Altiverb for orchestral-type stuff, and Space Designer for other things, or if I need a cheap & cheerful reverb for something that doesn't need to sound super-realistic. For authenticity, Altiverb wins, hands-down. Just get it, and you'll soon be saying things like "it's the best purchase I ever made." GREAT tool.