The "WD RE4 2TB", i.e. WD2003FYYS, at 115.9 MB/s average read rate, is the fastest HDD on that list.
The "WD RE4-GP 2TB", i.e. WD2002FYPS, at 87.7 MB/s, is somewhat slower indeed. As a Green model, it's designed to be quieter and cooler and consume a few watts less (IIRC something like 6W instead of 11W), rather than go for maximum speed.
Personally, I'm planning to build an HTPC with a WD2003FYYS for the O/S and the most often accessed stuff, and then add several WD2002FYPS (or some cheaper WD20EARS disks).
AIUI, some important factors to consider when assessing a drive's suitability for RAID are whether it supports TLER / ERC (Time Limited Error Recovery / Error Recovery Control), whether it is affected by APM (Automatic Power Management), and whether it supports the HDD Activity signal on pin #11 of the SATA power connector.
TLER / ERC determines how a drive handles read/write errors. It does this by imposing a timeout limit on reads and writes so that a drive won't be dropped from the array if it gets stuck on a bad sector.
APM causes a drive to go to sleep after a certain period of inactivity. Waking up from standby can take several seconds, which may impose an unacceptable lag on data transfers.
The activity LED is not always implemented by the HDD manufacturer. In fact I have seen one Seagate model which didn't support this signal in one particular firmware version, but did support it after a firmware update.
In short, it may be that WD and Areca have tested different firmware versions and arrived at genuinely different conclusions.
As for mixing drives of different speeds, I think this is OK, but it stands to reason that your RAID's overall performance would be limited to that of the slowest drive. In fact some people suggest that mixing drives from different manufactures or different families potentially improves the reliability of the RAID.
The difference is power consumption, and it goes parallel with you usage requirement! WD2002FYPS supports "IntelliPower Technology" which allows it to shift speed gear between idle and busy mode (5400 - 7200), which in turns logically reduces certain amount of average data retrieval speed but saves reasonable energy and adds life, where as WD2003FYYS is a work horse (7200 sharp) no matter what situation runs in hard enough win the race