I just want to stress again what I alluded to earlier, which is namely that 7200 vs 5400 rpm is not the only factor in drive throughput. I'd be inclined to assume that an older drive had 1) less cache, 2) very likely greater seek times and 3) was probably significantly smaller - all of which would tend to decrease performance in comparison to a newer drive for an identical workload. (Not to mention the fact that if you transferred data from one drive to another, that often tends to do a better job of defragmentation than most defrag tools, often resulting in an immediate performance gain even on identical hardware.)
However, assuming the same amount of cache, the same interface, and roughly the same seek times and size, a 3-5% average performance difference is likely to be in the ballpark for general use.
I use the faster drives as well - but since he seemed to be struggling to make the decision, I felt that numbers might help.
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