I don't buy it. Show me a comparison of boot speeds, then I am convinced.
The 500/5400 will come pretty close to the 320/7200 in best-case transfer speed tests, but it will be much slower where it really matters which is small random read/write. Not a mystery it's just that it has faster seek times because it's spinning faster.
Anyway, why compromise, just get the 500GB/7200 drive from Seagate = one to rule them all
Edit: To elaborate, the more capacity a drive has, the more stuff goes by under the read head in one rotation => therefore, higher data transfer rate. However you will see this only when editing or copying large files. For small files, random read rate is much more important because what takes the most time is not reading the data but instead finding the data in the first place. Here, 7200 drives are much faster because of their higher rotational speed.
If you are skeptical about any artificial benchmark tests like me, you can see this in real world when copying a folder with many small files in it. Let's say you have one folder with 100 files and each file is 10MB. That's 1GB of data. Copying that will be as fast as your HD can go, which is something like 50MB/s.
Now if you have a folder with 100,000 items and each item is 10KB or 0.01MB, it's also 1GB but if you copy it you will see your transfer speed drop to anywhere from 500KB/s to 1MB/s. It's dramatically slower. A 7200 RPM HD will still get 1.5MB or 2MB at least. A SSD will get much more, though I don't have one yet so I don't really know about the real world numbers.
Booting is a good test because it loads 100s of thousands of OS files. Real world it depends on what you are doing but stuff like starting applications is based in small random read speed. RAM also makes a difference because OS X disk caching is excellent and will compensate for the slow hard drive on many occasions.
I know all this because I have iPulse running and whenever I have a longer lasting disk operation going, I watch the average/min/max transfer rate