Rats, like with the GT-3, most of the modulation effects are put together in 2 blocks. 1 Block consists of effects like Acoustic Sim, Feedbacker, Slow Gear (=Auto Swell), Tremolo, Pan... The other consists of more spacy effects like Humanizer, Pitch Shifter, Auto Wah, Sub EQ, Flanger, Phaser etc.
So you CAN combine a tremolo with a flanger, but not a flanger with a phaser.
I've gotten pretty good results with direct recording out of
my GT-3 (Left Out to recorder, right out to amp, micing amp too) and those sounds were better than when I tried the POD on several occasions. The POD does win for ultra hi gain sounds (like Soldano and Rectifier), but the GT-3 (and probably to a better extent the GT-6 too) have very nice clean and crunchy sounds.
Oh another big thing is that the most important effects for guitar are always independant and simultaneously useable:
-Chorus
-Distortion
-Delay
-Reverb
and I believe EQ and compression too. This is an EXTRA EQ next to the amp sim's EQ. You can even quadruple the EQ, by using
1) the amp sim's EQ (low, mid, high, presence, bright on/off)
2) the EQ effect (low, low mid and high mid fully parametric, high)
3) Sub EQ effect (same as 2) )
4) Global unit's EQ (only low and high, but easy for on the fly tweaking for another amp or something when gigging)
The units (GT-6 and GT-3) take A LOT of tweaking to get a good sound, but once you get the hang of it, it's fine. It took me 5 years to make one really ultimate patch (I used the Soldano sim with a lot of mid boosting and half gain, to get a great allround metal/rock lead and rhythm tone. If I had kept the GT-3, it would be because of this very patch.
Also, the output can be configurated to send it to a mixing desk, a guitar amp input, a guitar amp's fx loop, be sure to get used to its sound. I didn't, and I always liked the Line Out (headphones) sound (which was made for recording). I got a lot of complaints when playing live that my guitar sound was a bit dull.
Now I use stompboxes live, so I'm used to a better response and dynamics again.
Bottom line:
if you need a do it all machine to gig with and record with, the GT-6 seems like your best bet. Be sure to also check out the Digitech GNX2. I like the Digitech's amp sims better, but the GT-6's effects can be tweaked more.
They both have stomp box modeling.