There aren't really any pros or cons for which drive to replace, it really comes down to personal preference. For example if you wanted to copy a DVD on the fly, then you'd need to keep the DVD Combo drive. On the otherhand, if you only copy CD's then the CD-RW drive is suitable.
Fitting the new drive should be fairly simple, all you need to make sure is that the jumpers on the back are the same as the drives that is removed - more than likely set to Cable Select. Then its just a case of plugging in the IDE cable and power - and possible the Audio cable thingy. When you boot in Windows should find the drive and install the drivers for it.