I also replied in the other thread:
I prefer alpine because you get a lot of features for it's price tag. With the JL you don't get much but reliability and name brand status. Alpine gives you both those as well.
There are 3 major features that really make the alpine worth it's money:
1. 150rms x 4 at 4ohms and 2 ohms! It's also under-rated, (as with many amps in this class). Some people report great underatings, mine was more modest at 164rmsx4
2. D-class efficiency - good for the cars charging system. It may save you money from having to upgrade to a high-output alternator.
Other companies are jumping the D-class multichannel band wagon - eclipse, rockford, etc.. but, from what I recall, their power to size to price ratio is still far behind Alpine's..
3. Form Factor - Clean wire terminals and a clean look. Stackable, a testement to it's well thought out design(form&function). Compact size. It's a definite space saver, which is so important for small to midsize vehicles and trucks. It is also great for stealth installs because you can hide that thing easily.
So yeah, get the Alpine if the above factors are important to your install.
edit to add:
oh yeah, there's also the alpine pdx 4.100 which is cheaper than the 4.150
and not much more than the JL.
Also, if you don't care about size and power efficiency, check out more midpriced amps such as pheonix gold, kicker, and hifonics.