Why SIP is "the next big thing"? compared to MGCP and Skinny.
well for starters SIP is IP based (and thus easy to integrate with existing internet services such as presence, voicemail, email and multimedia collaboration) and open standard, and is very easy to troubleshoot. SIP as it is based on IP, makes the requirement for expensive E1's/T1's and dedicated TDM equipment obsolete, by using SIP Trunks to providers, SIP trunks are also much easier to dimension.
SCCP, well it is cisco proprietary and thus almost per definition not supported by other vendors.
My opinion of MGCP is, that unless you would want a more centralised point of administration/call processing; use it. This because if you combine CUCM with MGCP gateways most of your administration and call control lies with CUCM. the thing with that is, is that in case of fail over (be it to SRST or CME) you will still need to rely on H323 (or SIP) anyway, so I dont consider it less work to configure.
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H323, SCCP and MGCP are IP based as well, perhaps you meant to say that SIP is based on HTTP.
As to when to use one over another it all depends on several factors such as preference, integration points, features required and supported as not all protocols support the same feature sets. Keep in mind that the beauty of having call processor such as CUCM or CME is that they can interpret all these protocols and you can use all of them if you choose in the same environment without issues.