I would strongly recommend the Nautilus as being the most bang for your $$$. To go to a seat that is only a 5 point harness and not also a booster wouldn't be as cost effective at this point in my opinion. The Nautilus also has a really nice cupholder and 'junk holders'
I wouldn't recommend the Apex because it has to have a headrest/seat supporting it up to the child's ears meaning if it ever needs to be moved to a different car without that, you cannot use it.
Another nice thing about the Nautilus is that it has some of the highest top harness slots on the market
It will last your little boy a LONG time.
Also, your little guy could outgrow the seat before he hits the 40lb mark. Make sure the harness is above his shoulders and that his ears are not over the top of the shell. If his shoulders are above the top harness slot or his ears are over the top of the shell, he has outgrown the seat regardless of his weight.