The differance in price between the budget 5,400's and the 72's is between 10-15%, which for a medium capacity drive may be as little as $10 but the performance gain all round can be quite substantial.
Often the choice will come down to what kind of machine you are building, if you are building a Celeron, Duron based system you must have already made some budgetary/performance decisions, so it would follow that in this type of system a cheaper 5,400rpm device will surfice, this may even apply with a low end Athlon XP or P4 if speed is not an issue but if performance remotely comes into the equation then there is absolutely no dought the extra few dollars will make more differance spent in this area than any other.
IBM 7,200's are amongst the quickest, Western Digital (special addition 8mb cache) take the performance crown but are a little more expensive, seagate barracuda's V's are the quietist and I think the most reliable? and the rest very close behind.
WD's, Maxtor and fujitsu make the best 5,4's
I believe though the small differance in cost is money well spent.
Martin Replying helps further our knowledge, without comment leaves us wondering.