Well, console specs (kind of) don't really determine how good online is. It's mostly up to the netcode that the developer creates.
Online connections usually are one of the two types, delay and rollback. Most fighters we see use a type input delay so the image is still smooth but the there is delay on the input. This is what we see in games like Brawl, Marvel 3, SF, etc.
Rollback is where the game records inputs and rolls back to hide lag. Since online connections are never fast and perfect, the game will be choppy at some parts in order to synch the players to the same time. This makes some characters "teleport". You'll see this happen on online shooters. Rollback is dependent on a console's specs, supposedly. Most companies don't choose to use this for their net code.
Rollback is better for the US since our internet is pretty crappy. Japanese developers will usually use the other type of netcode since their internet is structured better which is unfortunate for us.
So what is SSB4's online going to be like? I'm guessing not that good like most fighters. I highly doubt they'd do anything like a rollback netcode, so the connection will depend heavily on if you have a good upload speed.
As for which has the best online for console, either Marvel 2 or HD Remix, imo.
