From a capacity standpoint, if you can't afford the XL egg, get the 22" WSM.
If you're thinking more along the lines of Large egg vs. 18" WSM. Get the egg.
So basically, I prefer the egg but I did get 8 butts (62lbs) on my WSM yesterday.
I say just go buy either one because one day you'll probably end up with the other one someday anyway.
"I can get a good look at a T-bone by sticking my head up a bull's a$$, but I'd rather take a butcher's word for it". - Tommy Callahan III
XL Big Green Egg, 22.5 Weber OTS, WSJ Gold/Mini WSM, 18" & 22" WSM, UDS "No.8"
Flyers Orange Thermapen
Napkin Killers BBQ