I use both synthetics and the organic Iguana Juice. If I was wanting to stay organic I would stay with the Iguana Juice as it has performed very solidly for me in this grow of the one plant that I have in organics. I started out with this particular plant in organic soil that I mixed up and was making "tea" for watering but didn't like the tea I was making as it got messy and busy trying to do that and run my other hydro setup that is synthetic.
So I decided to get it simpler by going to just watering with the Iguana mixed into tea. The only complaint that I would have for the Iguana juice is that when it mixes up in a bucket, it has to be allowed to sit and aerate for 24hrs before adjusting the PH, and then another 12-24hrs before making final PH adjustment and adding to the recirculating auto-watering system that I set up for it. Once I have the PH set, it stays right until the water is used up and I have to add another bucket.
As for the pros of using it, the plant has stayed healthy and has grown big enough to take up 1/3 of a 5x5 tent and it is smelling incredibly good, and has nice Beasty buds that are about 2 weeks from harvest. The Iguana Juice doesn't stink up the place like the organic tea that I made before, and it flows nicely through my pump to the plant.
I also use synthetic nutes in the rest of my hydro setup. I use Advanced Nutrients' Jungle Juice and really like it. My plants are doing great on it. The only con for the Jungle juice is that it has no PH buffers in it and that makes me have to adjust the PH of my hydro resevoir about every couple days to keep the PH in the sweet spot. I would imagine the GH 3part doesn't have to be adjusted as much as it has the PH buffers. I plan to continue using the Jungle juice though as I like the fact that when I mix up a bucket of nute solution, it goes straight to 6.0-6.1 without any buffers, and I can pour it right into my rez without having to let it sit and aerate and then adjust again.
There's going to be good and bad in whatever you choose, so you just have to give it a try for 1 grow and see how well it works for you, then if you don't really like it, try something else on the next grow. I tried a couple different ones before I settled on the jungle juice.