What’s the difference between a steam and a hot water radiator?

Why did contractors specifically seek out hot-water radiators for two-pipe steam?

I have two-pipe steam in my house and a mix of steam (bottom-nipple) and hot-water (top/bottom-nipple) radiators, though mostly hot-water radiators. After living in the house for 12 years, my observation is steam radiators are superior to hot-water radiators for steam heat, the reason being in the shoulder seasons during shorter cycles, the smaller volume of steam in the hot-water radiator cools off from the cold iron mass before leasing its full heat potential into the air. By contrast, in the steam radiators, only the first few sections heat up, segregating the steam from the rest of the cold radiator. This also plays out as I insulate the house. I may have improved heating efficiency by 15% or more -- not enough to replace all the radiators in the house but enough to reduce the number of times the steam fills the oversized radiators. The segregated sections of steam radiators allow for shorter cycles of the boiler without compromising on performance. Just my humble observation...

Radiators connected across both the top and the bottom can be used -- quite successfully -- on both steam and hot water, with much the same results (except that you'll get a lot more heat out of the radiator on steam!). The problem which @motoguy128 mentions can be essentially eliminated by placing the vent no more than half the way up -- preferably no more than a third of the way up. The problem doesn't exist on two pipe steam.

The 3 mentioned are radiators 4, 5 and 10 out of 15 on the header.  They start heating at the same time.  No difference in evenness whether all TRVs are calling upstairs or not.