Reading through alot of this thread, here is my take on it:
Why Smash4?
Smash4 is a new game. That means it generates hype and it will generate hype. Smash4 Streams are way more popular on Twitch currently (mostly becuase it wasnt out and people tried to get a peak), but also because many many many players who are neither into smash or any other fighting game will pick up the new game, play it for a while, try their hands at a tournament, and then might (maybe they will all stick around, very unlikely though) disappear.
--> Smash4 at EVO will draw a huge crowd, quite possibly bigger than Melee, with many players unfamiliar with the FGC. A chance for growth.
Apparently Smash4 is being pushed by Nintendo.
--> That means less struggle for EVO-Organizers, more prizes, more publicity.
Why not Smash4?
The Super Smash Bros. Series is not game designed for competitive play. To play it competitively, the community needs to create a way in which it can be played competitively. If that ruleset is not fully fleshed out, competition will not be as functional as it should be and would be for games designed for the tournament ground.
--> Smash in general needs an established ruleset to run in a competitive environment. Having a unsatisfactory Showcase at EVO may hurt Smash4 in the future.
Smash4 seems to be developing into a more defensive direction. Eventhough it is too early to make exact predictions about how the Gameplay will develop, a general direction can be guessed. Defensive Games are not as appealing to viewers as offensive games are. Campy Fighting Games often get crapped on, defensive Starcraft Players are often less likable, and so on and so forth.
--> Viewability is being questions by some. Maybe just Melee fanatics, maybe not.
Why Melee?
Melee's scene is still more active than ever. It seems to draw in enough people to warrant it being on the main Stage.
--> Predictable number of Turnouts
While some Players may be bored by Melee, new techniques are being implemented into tournament play regularily. Shielddropping comes to mind. Matchups are still changing and new players still manage to showcase and master characters previously left unnoticed (Armada, Axe, Amsa come to mind)
--> Gameplay is not yet stale, developements can be seen
Why not Melee?
CRT-Troubles
"oldness" of the game. Barely any Profit can be had for companies involved. Viewership on old games are usualy dwindeling. There isn't alot of ways to get new Players. Melee had a Boost through Brawl-Newcomers and recent developments, but over time, it will fall of again, if the communiy doesn't grow in some form (new Smash Games, new Smash media, new Smash tournaments)
Supporting Melee might actually support a seperation between the Melee and Smash4 communities.
Conclusion
From a Business point of view, picking up Smash4 is the profitable thing to do for 2015 and will offer much more exposure than a repeat run of Melee would. No predictions about Longterm Profitability can be made yet, but Online Support makes a strong case towards longveity, at least for a couple of years.
From a competitors point of view, I am concerned about wether or not Smash4 will be ready to be presented, in a competitive setting, to the rest of the FGC and to passive bystanders. As previously mentioned, Smash is different, and needs a Set of Rules to make a viable competitive game.
I would love to see Melee make a return, since Melee has proven to be around no matter what. I would also love to see Smash4 get the respect it deserves and I hope cooperation with Nintendo will do the competitive Smash scene well.
In that sense, I would love to have both games at EVO, since they are indeed very different from the way they look and play out. The only realy similarity is the unique style of the Kill and Damage (and Combo) system, which will always stay true to smash (but has been used by others (PS:A) aswell) So why not expand on that category of fighting game? In the future it might not only be Smash anymore that works with a damage meter that goes upwards and a flexible combo game.
If that is impossible, i have more concerns about Smash4 making a succesful event than i do with Melee, which means i would not mind picking up Smash4 in 2016 instead of 2015.
Please help me find more arguments for and against the inclusion of either game, i will try to add to this post and hopefully flesh out the conclusion.