There shouldn't be any need to stick with Hitachi. Do so if you are
comfortable, but provided you stick with the same interface drives from
any manufacturer should work.
Can't comment on costs, although Larry has pointed out what look to be
cheaper drives.
The faster the disk spins the faster data can be read from it or written
to it. So a 5400rpm drive will be approximatly 30% faster than a 4200rpm
drive. In my experience the performance bottleneck on laptops is often
down to the speed of the drive, so (cost issues aside) its always worth
going for a faster disk.
Your question asked about throughput when used internaly as opposed to
externally. By an externally I assume you mean connected via USB? I
suspect that if you had the same drive internally and the same drive
mounted externally via USB that the internal one would marginally out
perform the one connected via USB but I wouldn't have thought there would
be much in it as I wouldn't have thought that USB would slow things down
much if at all, but I've never been in a position to benchmark it.
Just done a google looking for USB vs SATA (don't know whether you have a
SATA interface), and according to
http://www.rt.db.erau.edu/655s08/655webUSBSAT/Griffis_655proj_USBvSATA_vfinal.pdf,
SATA is noticably faster than USB, but I get the impression that there are
a number of factors which come into play.
HTH.