Should I get a 10 or 12 miter saw?

Online Answer
Main Differences Between 10 Inch vs 12 Inch Miter Saw 10 inch spins faster, whereas the 12 inch miter saw is slower due to its heavy blade. 10 inch is cheaper due to its clear cut option, whereas 12 inch miter saw is more expensive because the blades have teeth and that increases the value of a saw.
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A 7 1/4" saw works just fine to cut material thin enough for the capacity it will handle. If you are doing a lot of trim on a job site they are lighter to handle than most 10" models and have a greater cutting width than a 10" chop saw.
Dropping the miter saw blade into the workpiece too fast is dangerous and will likely result in a rough cut or splintered wood. This is especially true when you're cutting thin, narrow pieces. Give the motor a few extra milliseconds to reach full speed, then lower the blade slowly as you cut..
Buy the table saw first, and build a crosscut cut sled, or buy one, and then you can do nearly everything the miter saw can do. Go with the table saw. A table saw will do anything a miter saw will (with the help of a sled or miter gauge), plus you'll be able to rip boards to width as well.
Main Differences Between 10 Inch vs 12 Inch Miter Saw 10 inch spins faster, whereas the 12 inch miter saw is slower due to its heavy blade. 10 inch is cheaper due to its clear cut option, whereas 12 inch miter saw is more expensive because the blades have teeth and that increases the value of a saw.
You won't get through a 4x4 in one cut with a 7-1/4" (max cut size will be less than (blade size - arbor size) / 2 = (7-1/4 - 5/8)/2 = less than 3-5/16"). It's not just a 4x4 you can't cut.