WARNING!!! I am totally biased for AGP so take this for whatever you can get out of it. Could be wrong on some points, but I'm pretty sure.
First of all you are comparing IPS entry level 800whp kit to AGP's full 1000whp kit. Keep in mind when you add fuel, ceramic and powder coating to the IPS entry kit, it's is $650 more than AGP's full kit. And even then still doesn't include as much. What you are getting with the AGP kit is more of an OEM quality kit where you get more for le$$ because the volume manufacturing. AGP has built and shipped thousands and thousands of turbo kits for many years and is in a manufacturing position to invest up front to produce what would be a $12,000 kit, but for much much less.
Differences in the kits-
Like camarostar2010 said above, his IPS kit hangs below the frame of the car and can be problematic on rough roads, especially on lowered cars. The AGP kit is all tucked up higher, nothing hanging lower than the frame.
AGP kit comes with a little bigger, higher flowing turbos w/ 3" v-band outlets that help them spool faster and make more power than the smaller Garrett's in IPS's kit.
AGP kit comes with the larger higher flowing piping and intercooler
With AGP you get a choice of two intercooler sizes, a big one or a HUGE one. Same price.
Cast iron manifold adaptors in the AGP kit are certainly an advantage for heat retention and durability/longevity.
IPS kit has air filters much lower to the ground, right on the turbo. AGP has short intakes that keep the filters up and out of any water.
AGP comes with heat shields, dual PCV breather/catch cans, ZR1 iridium plugs, 3bar map sensor, ceramic coating and powder coating all standard.
The AGP charge pipe has 4 aux npt bungs right before the TB for breakout IAT. Meth inj. etc
AGP kits have been shipping to dealers and fit right, bolt on easy and make power. IPS is still gearing up to be able to ship.
Not sure these details on IPS but-
no drilling or tapping any part of the engine for oil feed and return
oil feeds to the turbos are from a filtered oil source off the engine
oil return has secondary filter for the oil returning to your engine
Be vewy vewy quiet, I'm hunting GT500's.