I don't think you can go too far wrong either way. They're both wonderful. They don't really smell alike to me, but they do share that ample, strapping qualities of the masculines of their era (3me homme, 1985--bois noir, from which egoiste derives, 1987.) i know 90% of all basenoters answer to the either/or question is, 'Get both. And maybe these other 4 as well.' We're the admittedy enthusiastic.
3me homme is so inexpensive, like its siblings Yatagan and pour un Homme that i worry that i should hoard. Why are they so cheap? Should i stock up against discontinuation? The Chanel is bit pricier and harder to find, i believe.
My take on each. Egoiste is beautiful. A fruity, woody pseudo-oriental notable primarily for its richness. 3me Homme has a full helping of the coumarin darkness of the classic fougere, but the endearing beauty of a potent jasmine floral. 3me Homme is my favorite fougere ever.
All that is to say, if you can swing it, get both whenever you can. Of yeah, get Yatagan, too...