I use the Line 6 Podxt live, it covers pretty much everything I need (distortion, overdrive, amp simulator, chorus, phaser, flanger, wah, volume, compressor, gate, eq, boost etc.) There are a few pretty cool effects.
I have tried the Boss GT-8, all I can say is, don't get it if you're just a beginner at sound modelling. It is a very good multi EFX pedal, but half the terms used in modelling your sound, I don't know what it is. But it has MANY MANY MANY cool effects to play with, but I don't really need'em. Overall, this is one hard to tweak pedal, but if you have the skill, you can get a kick-ass sound.
Overall, I still prefer my Line 6 Podxt Live although it doesn't have all those cool effects, and less features, but I dunno, thats just enough for me, I don't really need those others that the GT-8 has. And the Line 6 is much cheaper used. I also appreciate Line 6 for their ease of getting a nice tone. You just go, select a nice amp, tweak amp eq, select a stomp box, maybe add some chorus or flanger, delay, that's it, you get a really kick-ass tone. But if you are really skillful in tweaking and modelling sound, the GT-8 is a really good choice as you really get to tweak every single detail of your sound, the parameter (I don't really know WTF is that, I thought it was a pretty cool term while I was being given a tour of the GT-8, someone enlighten me) They have much more intricate stuff that you can tweak, to get the sound.
My advice, assuming that you are new to this, is to get the Line 6, its really much easier when you get the hang of it, there are many programs and you also can download your favourite tones on the internet, use the Line 6 Edit to edit your tone with ease. Line 6 Monkey to update your drivers, and they may have a few freebie packs. You also can do USB recording, many cool computer-interactive stuff to do to tweak your tone with ease.
My $1.00 worth.
Cheers,
WWH