What is the saying, " 9 times out of 10 you should automatically bat, the other time you should think about it and than bat anyway". or something along those lines.
I would say even though it may not be the best pitch to bat on, they're most likely going to experience worse batting conditions batting second. Plus with the wet, although the outfield will be more slow, their bowl will die quickly and not do much.
Batting second puts so much pressure on your batting, especially your second half of batsman. Needing 50 odd runs with 5 wickets in hand can seem a long way away when the pitch is playing up and the pressure not to go out is on.
Runs on the board is the key !
I have had almost no experience with being a captain though.