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Ran,
The major difference between the two is that Cramfs is a read-only file system while ubifs is a R/W file system. The motivation of using cramfs is whne you want to have a basic files system which is left intact and your system can always mount to it as it does not provide write permissions.
In terms of performance the question would be if your file system is placed in NAND or RAM, this can be handled through the bootargs and bootcmd commends in u-boot.
The following links will assist you with more information.
http://www.cyberciti.biz/files/linux-kernel/Documentation/filesystems/cramfs.txt
http://www.linux-mtd.infradead.org/doc/ubifs.html
Yair