Ring. I'm not really a necklace person. I also can't say I consider spessartite to be all that delicate, no more than any other garnet. They have a hardness range of 6-7ish which anything in that range is fine for rings as long as you treat the ring well. If you're wearing said ring and digging ditches, you might end up with a problem...lol
Im usually a ring person because I like to be able to admire it while wearing! In this case I think the pendant is executed better and it looks really dainty.
I love the Tiffany firefly too. I would choose pendant because I just don't think I would wear a ring this whimsical very often. However if you want to be able to enjoy looking at it, by all means get the ring!
Difficult position to be in. The pendant seems more suited design wise but I rarely wear necklaces, plus I can only admire it from mirror reflections I generally prefer rings so that I can admire it all the time but the ring design doesn't speak to me..