As far as heat goes, watts are watts. A 5000W unit heater will give 200W more heat than a 4800W construction heater. That's it.
But if you're talking longevity, no question a dedicated unit heater will outlast and be safer than a construction heater.
Here's a safety notice put out by our dept of labour:
http://www.gov.mb.ca/labour/safety/me/pdf/me021.pdf
For once in a while use, a construction heater is fine. For season long heating, I'd use a dedicated unit heater. I know others will pipe up and say "I've used construction heaters for years without a problem." and I hear you, but I personally wouldn't do that.