I have had both types. I started with the Fimco 3 nozzle boomless, that went to a single flood spray nozzle and now I am using a 10' 5 nozzle spray boom on my ATV.
Each has it's advantages but the boom sprayer with multiple nozzles in my experience is superior in coverage. Boom-less sprayers shoot out larger droplets in order to get coverage and your spray is coming in at an angle. When you use post-emergent herbicides such as round-up, If the chemical does not touch the plant, the will not die period. Boom-less don't offer as uniform of a coverage and the outer limits of the spray width the weeds are being shielded by other weeds.
I very rarely use my boom-less nozzles anymore except for the occasional times I spray fertilizer. Maybe others have had better results than me but every time I used one for a weed burn down I would have to go back over it again in a week or so and spray again because of missed plants. With my spray boom I don't have to deal with that anymore.