Well from the size difference I'd probably take the Seagate but if it doesn't work fully 100% with everything you need then it's worthless to keep for this purpose.
The WD is USB powered so is easier to use and hot swap between your Wii and PC. I do find the WD is good with everything and very quiet and cute looking.
Up to you really. What do you feel is best?
As for splitting the partition I couldn't find a way so copied all ISO to my PC, then formatted in windows and then partitioned into two drives, the first of which I made primary and active before converting it to WBFS. I forget which program I used but easeus is good apparently. I kept only 30GB for my windows volume and the remaining for WBFS.