The only real "advantages" the i5150 has over the i8600:
-> Has a normal aspect ratio screen as opposed to the widescreen on the i8600. To some people this is preferred (although personally I like the WS
-> Obviously, have more pure CPU power behind the hood. But expect the i8600 to take this mantle as well with the new Dothan 2.0GHz in the future, as mentioned.
-> Is smaller lengthwise, making it easier to fit into some bags (although it is still thicker and heavier than the i8600).
Well, that really is it. Build quality is basically the same as the i8600, keyboard may be a little more firm... but really thats it. The main advantage at the moment for the i5150 is the CPU power, but as this will be surpassed by the i8600 in the future there is no firm ground upon which I can recommend it.
The i8600 is lighter, has longer battery life, runs cooler, kicks butt in games, has a beautiful screen for DVD's, and has plenty of power for todays apps, with the addition of the Dothan CPU it will become a powerhouse in every sense of the word.
I know it sounds like I'm beating the metal drum here, but I say this in all honesty, not just because I own it.