They are all very similar but have some definate differences.
1) JL Cleansweep - This is what I have running in my car and its excellent for what I need. It has a remote volume knob, so you cannot use your factory headunit volume knob (you can but it defeats the purpose). It will give an entirely flat eq signal. So you will need an external equalizer, use your computer to equalize, amps, or what have you. It is a very basic, but very good solution. JL Audio is awesome. But I'm sure you know that. Only has 2 pre-outs though, front and rear. You will need to have your amps do all the HPF and LPF.
2) The Alpine PXE - I'm not a pro on this one or the rockford, but I've got some research on these through my legacygt forum. The alpine is a good option and I think this one allows you to use your headunit volume knob, but I think this one also lacks an equalizer. If I remember correctly.
3) The RF 360.2 - From what I've read this is the most popular one because it has an equalizer and produces better bass response. Again, not 100%, but thats just what I remember about it. It uses a remote volume knob so you wont use your headunit volume knob. I believe this one also has 3 pre-outs for your setup (front, rear, and sub).
They all have disadvantages and advantages, but for the most part they are all very similar in that they take your factory headunit signal, convert it to a better signal, have outputs for your amps, have at least 1 input for your iPod, carputer, what have you...
Hope this helps.