I'm sorry, I can't offer any comparison. But I can give you a little info, maybe draw more interest to your thread.
There is at least one guy on here doing an all flouro grow, look around.
I, and many others, use flouros for delicate seedlings and clones. Work well for mothers, to slow growth a little.
As far as general cheap bulbs, 2x 4 foot 40 watt flouros will put out 6000+ lumens, not bad for $15 (shoplight fixture). But if you have alot of space to light, a 1000 watt mh puts out 110,000+ lumens, so if it takes 40x 4 footers to hit that, you are now burning closer to 3200 watts, plus initial cost is higher.
Flouros also run MUCH cooler, requiring much less extra equipment. HID bulbs need changing at least once a year, flouros last much longer.
So if your veg room is small, or cooling is a big problem, flouros could work out well. Get 6500k (mh color) or 3000k (hps color) or mix like I do.
As far as flowering though, HPS is alot more efficient than MH, giving closer to 130,000+ lumens per 1000 watt light...I don't think you can get near that kind of efficiency with flouros.
So, in closing, I think it really depends on the size of your room, access to cool air, size of wallet.
I am really surprised more cabinet and closet growers don't use them. I think 10 flouros (at around 3100k+ a peice) would draw as much as a 400 watt MH, with nearly same output. But here again, a 400 watt HPS is closer to 50,000 lumens (more efficient).
If you decide to go flouro, there are some other tricks you could utilize (if you are slightly mechanically inclined). If your space is REALLY small, you could overdrive the flouros and double each individual bulbs output, but you will generate more heat and lose efficientcy. You could also remote mount your ballasts to lowr temps even more.
I hope some of this info could be useful to you, I have not finished my coffee or had my morning smoke yet, so I hope my math is okay.
Good Luck!