The mistake many people make is that they don't put things into perspective and consider context and comparative nature.
The IG is a tool. The Phoenix Force is a sentient entity with its own agenda.
To simplify things, lets equate a potential user of either to a learner driver.
The IG would be an automatic Ford family car. The Phoenix Force would be a manual sports car.
The IG is easier to control and gives easy access to its full suite of features from the get go for just about all users. Leading to dramatically less variance between user showings.
The Phoenix Force however is a far more temperamental beast to tame and what you can get out of it depends on the users skill level.
Because of this you will always get consistently high showings with the IG.
With the Phoenix however it depends on the host, were they already an adept energy wielder or telepath, are they emotionally stable, is what they're trying to do aligned with the Phoenix Forces agenda (if not then the Force will simply turn on you and limit what you can do as seen in Uncanny X-Force when Jean attacked Death/Archangel)
However if you have a competent user, who has the necessary willpower/emotional stability and they're carrying out the Forces agenda then the feats such hosts have achieved have been beyond the best showings of the IG by a considerable margin and that's not debatable.
A manual car is tougher to master, but gives finer control and enables you to eke out performance that you never could from an automatic.