The difference in wheel size will change the gear ratios so with the equivalent crankset and cassette the 26 inch wheel bike will have "taller" gearing.
The larger wheels will also roll over curbs, roots, etc a little easier than the smaller ones.
The 26 inch wheel will give the bike an extra inch of bottom bracket clearance, assuming that the frames are similar.
The 24 inch wheels will be imperceptibly lighter than the 26.
It is easier to find 26 inch tubes and tires than 24.
The 26 inch wheel bike is likely to have a slightly taller standover and possibly a longer wheelbase as the chainstays need to be a bit longer.
The resale value is based more on the type of bike and components than the wheel size.
If you remove the standover height from the equation then the wheel size is irrelevant when it comes to which one you would outgrow quicker. The way you fit on a bike is more related to the length of the top tube and head tube than the wheel size.
Other than the above, there is no reason to believe a 26 inch wheel bike will have better components than a 24 inch wheel. In fact there are many dirt jumper style bikes that use disc brakes where they come with 24 inch wheel but can accomodate 26 inch.
Go to some bike shops and have your son sit on both some kids specific bikes and then on some XS frame size (usually 12.5 - 14 inch) frame adult bikes and see how he fits. I will be in the same boat with my son in a year or so, my daughter saved me the grief by shooting up like a weed so she was easily tall enough for a small adult bike after growing out of her 20 inch wheel bike. I have been looking at kids hardtail mountain bikes/dirt jumper bikes as that is what suits my son's riding style. In my opinion there are very few good kids specific bikes out there as most people are not able to justify spending 500 plus on a kids bike. This makes the small frame size adult bikes a lot more appealing if they fit. There are some high end kids bike if you can justify the cost. Notably Norco Kompressor, Norco B-Line, Kona Stuff 24, Kona Stinky JR and Specialized Big Hit Grom. Alternatively there are some much cheaper extra small adult bikes such as the Gary Fisher Opie.