To state it simply, I would buy a 10" Hitachi or Makita sliding compound miter saw. These two are consistently the most enduring and favored saws I see in the field as a professional. This statement is in total disregard for price if you are on a budget.
Bosch has a good product and DeWalt's saw has had a lukewarm reception among the contractors I know.
Now for the long answer:
I would choose a 10" because they have nearly the cut capacity of the 12" and they tend to have a more stable cut because there is less distance from the arbor to the tip of the blade to allow for blade deflection.
10" blades, generally speaking, are less expensive to buy. Blades such as the CMT General can be used on both the chopsaw or the tablesaw with excellent results.
The CMT General happens to be one of my favorite blades because it gives an excellent cut either way and it is only a 40 tooth blade. This blade defies the common thought that a high tooth count is required for a clean cut. This blade gives me exceptional cuts and with only 40 teeth costs less to resharpen than a 60 or 80 tooth blade.
I own 3 saws and my favorite is the 10" Hitachi C10FSH. It is incredibly accurate and I like the laser for quickly finding where to locate my cuts.
The 12" DeWalt DW 705 is a good workhorse. It is accurate and lightweight so it goes to the field the most.
The Makita goes to the field when I am cutting a lot of material like shelving or siding because of it's wide cut capacity.
Concerning DeWalt: I cannot speak so much for the new generation of sliders but like I mentioned the reception has been lukewarm for them.
Personally speaking on DeWalt, I had purchased the sliding compound miter saw and the dual bevel chop saw and returned them both. I started with the 12" sliding compound miter and could not tune it to create a square box so I returned it. I bought the 12" dual bevel miter saw and had the same issue. But I have had great service and accuracy from my DW705.
I had used the Hitachi 10" and 8" sliding saws in the field and my brother's shop and was always impressed with the stability and accuracy. Even more impressive was how old some of these saws were. That was what drove me to purchase the Hitachi and I have not regretted it.
My opinions and observations are formed by what I see and experience in the field as a professional. They are not by controlled conditions and side by side comparisons measured with micrometers and digital equipment.
It is common practice for contractors to share another's equipment such as a chopsaw or tablesaw when they are already set up on site. One thing that I notice from this is that a trim carpenter is more finicky about his tools than a framing carpenter and this would be obvious. But both situations can give valuable information on the durability and accuracy of equipement.
Buy the best that you can the first time, it will save you money in the long run.