TL;DR: buy a Powerball and your choice of a SLO, Uber, Savage, or 6505 depending on if you wants leads/80s tones or brutal death/numetal tones.
If you want a clean channel, the Powerball is the only high gain module with a great clean and rhythm channel. If you need to keep your module count down, it's pretty much a required module IMO. You could buy it and a lead oriented module like a SLO and be set. Or you could use the 2nd channel for leads and buy a Savage or Uber for your rhythm tone.
My preference is the Savage. I also use the Powerball and 6505 a lot. I'm still undecided on the Uber, but it is more unique than the others. The IICP is good, but it's pretty much a single channel module due to the gain stacking and eq. The SLO is good for 80s rock, but I haven't used it much since the 6505.
I also have a GPDI and it sounds very similar to the GP3 / pitbull. I personally think any of the high gain modules will be better than a Pitbull. I definitely suggest you play the Fryette pres in person before deciding. They are pretty much love/hate tones.