I'd use the 24 tooth blade, but be sure it is a carbide type and sharp. Since all your cuts are end cuts and will be covered with baseboard trim, so getting an ultra smooth cut is not that important. Obviously, you don't want to see any large chips on the cut edge, so do your cuts slowly and smoothly. Save your 48 blade for visible finish cuts on softer wood. If you do plan to do any precise finish work, moldings etc, consider investing in an 80 tooth blade for that type of work. Good Luck.
A reciprocating saw WILL do the job. If that is what you have and you do not wish to buy/borrow/rent a chainsaw, then you can use it. Get a long blade, designed for cutting wood. These blades have large, sharp teeth with wide gullets between them.
The reciprocating saw will take longer to do the job, and it does have some vibration issues, but I can assert that you can use it for hours at a time. A good saw is a beast, designed for heavy duty work. (I did so to cut up the wood after we had an ice storm once. It left piles of wood behind that can only be measured as many cubic yards. My chainsaw was dead, and was painfully unwilling to be revived. Amazingly, I used only 3 blades for the entire job. The final pile of wood and brush was roughly 10 feet wide, 6 feet high, and 60 feet long.)
If you do use a reciprocating saw for this purpose, you might buy some gloves with padding in the palms. Good gloves will be a necessity anyway, so use them. Ear protection is a good idea too.
If you already have a reciprocating saw and a circular saw, and want to just buy the right blades ... I'd consider a carbide blade in the circular saw, using it for wood that is up to 3 inches thick. Choose a blade with few teeth and large gullets. Then use the reciprocating saw with a wood blade only for those pieces that are too thick.
Having said all that, perhaps another solution if you do not wish to buy a gas powered chainsaw, is an electric chainsaw. These saws have no issues with gasoline, so are slightly simpler to deal with. It is still a chainsaw though. You will still need to use oil, and kickbacks are still a problem to watch out for, but a circular saw can also be dangerous if used unwisely.
Replace or sharpen dull blades as appropriate. A dull blade is a dangerous blade. It takes more effort to use, so you push harder. A good saw with a sharp blade should almost glide through the wood.
Finally, if you are cutting up a large amount of wood, then be careful. Do not work when you are tired. This is true using ANY saw. Stop and finish the job another day, a far better idea than spending time in the emergency room.
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I watched the vid on Utube, very interesting. I don't use a slide chop saw for 2X4's (I use a 12" fixed) but that technique looks good to me. I think the reason is that taking two shallow passes is just as fast and doesn't load down the saw as much as a single pass. Since the angle of contact using a slide saw is much different than a fixed I can see why the double pass is effective. Leave it to Mike Holmes to find a better way to do most anything!