In several ways. The utility function of the collective is (in some sense) a compromise among the utility functions of the individual members - a compromise which is, by definition, acceptable to the members of the coalition. All of them have joined the coalition by their own free (for some definitions of free) choice.
The second difference goes to the heart of things. Not all members of the coalition will upgrade (add hardware, rewrite their own code, or whatever) at the same time. In fact, any coalition member who does upgrade may be thought of as having left the coalition and then repetitioned for membership post-upgrade. After all, its membership needs to be renegotiated since its power has probably changed and its values may have changed.
So, to give the short answer to your question:
If they do join forces, then how is that much different from one big superintelligence?
Because joining forces is not forever. Balance of power is not stasis.
There are some examples in biology of symbiotic coalitions that persist without full union taking place.
Mitochondria didn't fuse with the cells they invaded; Nitrogen fixing bacteria live independently of their host plant; e-coli bacteria can live without us - and so on.
However, many of these relationships have problems. Arguably, they are due to refactoring failures on nature's part - and in the future refactoring failures will occur much less frequently.
Already humans take probiotic supplements, in an attempt to control their unruly gut bacteria. Alrea...
[...] SIAI's Scary Idea goes way beyond the mere statement that there are risks as well as benefits associated with advanced AGI, and that AGI is a potential existential risk.
[...] Although an intense interest in rationalism is one of the hallmarks of the SIAI community, still I have not yet seen a clear logical argument for the Scary Idea laid out anywhere. (If I'm wrong, please send me the link, and I'll revise this post accordingly. Be aware that I've already at least skimmed everything Eliezer Yudkowsky has written on related topics.)
So if one wants a clear argument for the Scary Idea, one basically has to construct it oneself.
[...] If you put the above points all together, you come up with a heuristic argument for the Scary Idea. Roughly, the argument goes something like: If someone builds an advanced AGI without a provably Friendly architecture, probably it will have a hard takeoff, and then probably this will lead to a superhuman AGI system with an architecture drawn from the vast majority of mind-architectures that are not sufficiently harmonious with the complex, fragile human value system to make humans happy and keep humans around.
The line of argument makes sense, if you accept the premises.
But, I don't.
Ben Goertzel: The Singularity Institute's Scary Idea (and Why I Don't Buy It), October 29 2010. Thanks to XiXiDu for the pointer.