Rental Checkout flight is a situation that should be relatively under control if the pilot understands the risk factors and is confident of his/her skills and experience.
In practical terms though, you should be prepared upfront for such an engagement that encompasses both a ground and airborne component. Do your homework accordingly because everything can be covered.
If you're experienced flying C150 and want to rent Skyhawk 172SP with G1000, I would say that you would have to go through a transition training. A checkout would not suffice in this case.
Bottom line is, be clear of your intentions and understand the aircraft performance features, airspace requirements and regulatory compliance issues.
Be absolutely clear that you are proficient in operating the rental aircraft and you should be through with the checkout within an hour time frame.