You won't see much difference I am afraid, but I can see you are bound and determined
If I weren't, I would have given up on computers long ago. It is arduous path for a person like myself of limited financial means to attempt to get the most out of the least where computers are concerned. Unlike many, I never start out with buying a new rig, or all new components for one, I try to keep improvements as a continual series of steps with what I have. The computers that I have today are not what I started with a few years ago, but the transformation has been a slow steady change. I will probably always be well behind the state of the art, but I don't want to be left so far behind that my equipment becomes totally obsolete.
This post makes complete sense to me. The last few sentences are the real meat and potatoes of your argument. In the case of taking external drives, getting a new SATA III drive, putting it into a SATA III enclosure and connecting it to a dedicated stand-alone SATA III controller card to me says "I want to be at state of the art". Rest assured by sticking to a SATA II drive, you aren't being left so far behind to become obsolete...you merely have what most regular users have as well as most techies.
I'm a techie, I do IT for a living. Have been living with and working on computers for 25+ years, over 12 of which I have done professionally for a living. I generally build up a complete new computer every 3 years on average. I never buy state of the art, I'm always a couple of clicks down from the top...as it saves me a bunch of money and 2-3 years down the road that really high priced option is just as outdated as the few clicks down compared to what is available. And it's pretty darn rare for me to upgrade my system much at all. I might add a hard drive, or maybe swap a video card over the 3 years...but that's about it. My new computers have historically always been new cases, the whole shebang. This way I can give the old computer to my kids, make it a new server, donate it to a friend, etc.
I generally don't upgrade along the way to get things like USB 3.0 and SATA 6.0Gbps because the improvement is often very, very small at the start. I mean in all honesty, a standard mechanical hard drive in a SATA 1.5Gbps interface versus a SATA 3.0Gpbs interface makes almost no difference whatsoever. Consider that SATA 1.5Gpbs offers about 150MB/sec through the interface...I've yet to see a mechanical hard drive (even the latest SATA 6.0Gpbs drives) that can offer that. And now we have a new standard, SATA 6.0Gbps which offers 4x the potential speed....but I beg...what is the point? (understanding completely well that an SSD changes the scope entirely.)
I wouldn't think upgrading a mechanical hard drive to SATA 6.0Gbps with a controller card will net any more than 5-10% improvement. And even at that, I don't find that I am standing around waiting for hours on end for my file copies to my external drive to complete as it is. I could easily hit the restroom, grab a snack and everything would be complete.
Your best bet, is to slap a 10,000 RPM 600GB Raptor drive into a SATA II port. You would get a bit more speed, quicker spindle access to data elsewhere and faster random access time. I wholeheartedly feel this would provide more benefit than going the SATA 6.0Gpbs route with a typical 7,200 RPM hard drive.