Not much, if this is a budget question the cheap ones work as well, if your wanting the best go top brand Philips or something, a good compromise are the mid range.
I've used Hortilux in the past but I didn't notice any difference using those over the Vivosun bulbs. I can get a Vivosun 600 watt HPS for $17 while a Hortilux costs 4 times that much. You'll find some that will say you need to buy the most expensive of everything if you're serious about growing good weed. I don't agree with that mindset. The Vivosun and Ipower bulbs will grow excellent weed and the money saved can be kept in the bank to use for other things.
I never used a Vivosun like @xtsho , but I think I should have. Ran Hortilux Super 600 lots, a couple of Uisho super red Supers, and one phillips son-t-par non super. I liked the Phillips best and it was cheaper than the others, but still an $80 light. I was paying 140 plus for a hortilux super in 600 watt in Canada.
I will be running a cheaper bulb based on this advice. Hortilux bulbs seem to be going up in price guessing because of LED's taking over. I have to run HID in the winter. I'd love to just run CMH/LED but haven't quite pulled the trigger on getting the entire flower space switched to that.