If I'm doing a turn-based card game, what kind of technique do you use to support multiplayer gameplay over Internet?
Is it socket? If it's socket, which SDK (CoronaSDK etc.) can provide solid socket library?
Can Unity3D can be used solely to support what I need without using other socket servers like SmartFox or Electro?
If your game is turn based, a combination of long-polling (read Push) and web services (SOAP or REST) should be more than sufficient for any turn-based game. They are simple to implement on a server and consume on a multitude of clients.
The advantage of using web services, are two fold. First, just about every platform worth its salt will have a mechanism to invoke web services natively without much ruckus. Second, is that adding additional platforms, or even letting web (desktop based browser) players play against anyone else, is easy since you're just passing messages back and forth over an HTTP connection.
In addition to that, since you're talking about Android and iOS -- where 3G connections are likely to occur, an HTTP web service wont completely die in a "flaky-connection" mode, where as pure TCP sockets would have issues staying connected.
I had the same dilemma when I ventured into Android gaming. SmartFox was beyond my budget but I was keen to use a third-party SDK which I could leverage across multiple games. Further, I don't really have the programming knowledge to design a socket library interface by myself. I came across Skiller - a free SDK for developing turn-based multi-player games which integrates with Eclipse and abstracts such details from developers.
I'd suggest you take a look at Skiller's TicTacToe. It would give you a good idea of whether their SDK would suit your needs. I hope this helps.
