For someone with no restrictions on money or shelf-space, I'd say it was worth having both. If I could only save one in a fire, though, I'd have no hesitation in grabbing Alchemy Companion.
Hear hear! I've had Alchemy Companion since it was first released and had read through portions at a time and picked out the relevant bits I liked, but I had never read it through cover to cover. This weekend, I finally sat down and read through the book in its entirety. What a fantastic read! I'm sad that I didn't read through it many years sooner.
The world we run is an Elemental Companion only world. The only magic allowed is from that companion. This past summer, I offered the players the option to use Alchemy Companion as well since the setting is similar to the Renaissance Period when Alchemy was all the rage and mysticism and magic were fading away.
I like being able to read a book, not simply a PDF on a screen. I would say purchase the paperback version if it's available and cost effective. At worst, the PDF is good to have, but the book is well written and offers some great views on alchemy, some very cool professions, spell lists, and crit tables. It's definitely worth the purchase.