It is very easy to maintain heat on a WSM (I think the egg is even easier, with very little fuel, but I have not used one)
I can get the WSM to "dial in" to a temp, then actually leave for an hour or 2, come back and its still where I left it.
Again, not speaking for the Egg owners, but I think the capacity on the WSM is quite a bit more, even than the largest Egg.
Not too many downsides I can think of with the WSM I have. If you do full slabs of spares, you might need to "drape" them over something to get them to fit, or us rib racks. That might true for the Egg though too.
I would recommend the WSM for anyone, and have given one as a gift to the guy who got me going on my ECB years back