I went with the 5400rpm they dont get hot they run at a warm temperature which is good considering its enclosed. All my pc external cases 3.5" i even run WD Green 2TB drives they are 5400rpm i have ran 7200rpm drives in cases and 5400rpm drives and the heat difference is huge and the drive will last longer and perform better if it is not boiling to the touch. The speed difference between the 2 spin rates is minimal in read it is more noticable in write that is on PC external drives on E-Sata to a pc with raid drives that do upto 140mbs.
E-Sata speed 5400rpm from memory was around 60-90mbps, write = 60 or so read in the 80s. 7200rpm is around same in read get little more upto 105mbps on abgx but that was not the avrg id say write be in the 70-80mbps and read upto 100mbps. This will differ in drive types, file sizes, fragmentation of files and pc setup aswell but the 5400rpm drive is a better drive when you are running it in a enclosed space.
XBOX360 games run from usb with no problem so i do not see any point in adding a drive that may give a slight 1second or so saving on load times for games when a 5400rpm drive will perform just as well if not better as its not going to be cooking every time it is being used but this is my opinion and the best way to find out is test a couple drives on your pc in a external enclosure i did my tests last year but this was my results from memory. Use win7 gives drive speeds as transfers are running or you can use diagnostic testing software.