In a perfect world, should RIPE160 be used as the block ID or transaction ID? Edit: In this question I'm looking at storage advantages, and network bandwidth savings.  

Given that the TxID and BlockID uses SHA256 to uniquely identify a transaction (and block) is there any reason that RIPEMD shouldn't be used in a brand new/different alt-coin?
The malleability problem is not with the hashing function, but with what is being hashed (or not). The hashed data is malleable, not the hash directly.