The main criteria for selecting MGCP over H323 is the ease of administration of dial plan. With MGCP, you can do a centralized administration using Callmanager. With H323, you would need to do this at each gateway independantly.
When it comes to SRST, anyways you would need to setup the dial-peers for fallback purposes. If you have a straight forward dial-plan, then you will be better off doing it using dial-peers.
If your dial-plan is complex, then it would be better to use MGCP when in normal mode and you can use a simple dial-peer in fallback mode.
regards,
Anup