RAM compatibility between MacBook Pros

There's no benefit in going with faster RAM because the speed is limited by what your motherboard supports (the early 2009 iMac has a 1066 Mhz frontside bus.) In theory the faster RAM should be able to work at the lower speeds and should be safe to install, however macs do tend to be fussy when it comes to RAM so I would recommend sticking to the RAM recommended in the system specs.

Edit:

More often than not your mac will accept higher speed RAM. However it can cause problems; one example from the Apple Support Forums. You can find similar stories all over Apple forums. This is why I advocate sticking as closely to the specs as possible.

As to the advantage of possibly installing the RAM in a future computer, in 2 years you'll probably be able to get twice the RAM for the price (and be wanting to install twice the amount too.)

Lastly, if you compare the 2 chips you linked you'll notice that the 1333 memory has a CAS latency of 9, while the 1066 has a lower (better) latency of 7. So you're trading an imaginary upside for a real downside.

Since Apple is using the same CPU and mostly intel logic boards, as long as you get the CAS latency and other details exactly perfect, it's hard to imagine you would buy the wrong memory.

Perhaps it boils down to support and testing. If the vendor literally spends more money testing and supporting the Mac memory, it's their privilege to sell it for more. If they don't and are just looking to maximize profit, that happens too. Look at Amazon charging different price based on who they think you are, what you have bought, etc... Same with airlines that charge different prices based on cookies set from previous flight searches.

In the end - all that matters is you understand the return policy and how to test things once you get the RAM. Then it's just a calculation, you estimate how much is your time is worth and you minimize your transaction cost - pay more for an easy buy that saves you time or spend more time looking for and researching a bargain. Perhaps the seller has decided Mac users value their time more highly and you can score a deal by saying you are buying PC RAM.

Happy RAM hunting.