MH doesnt put off quite as much light as HPS but close (will be fine for veg)
A 5x5 tent with a 1000w HPS is good but a bit under powered if you ask me. Most ballasts will run at 1100 or 1150w which would be better. You can put a 1000w in a 4x4 though and that is really your ideal amount of light.
I run a 1000w double ended HPS with a sunsystem air cooled hood (the only hood on the market that will allow you to run a double ended bulb in a tent, any hood that blows air directly over the bulb is junk and will reduce output big time). Double ended HPS put out 20-30% more light than single ended. And i run mine at 1100w. So to compare my setup to the standard 1000w hortilux, its actually like im running about 1400w worth of light in my 5x5.