Ceiling height of 35-40 feet.
I was thinking about replacing existing 250 HPS (25,000 lumens) with 200 watt pulse start metal halide (20,000 lumens) in an open bay building or a t-5 HO (15,000 lumens)with 4 4-foot bulbs. What are the nominal fixture efficiencies? Which is best as far as color rendering index and energy efficiency for these given heights for equal footcandles.  

I Don't know much about T-5 it's new type of lamp and it operates real hot. I would use the metal halide but you would have to use more of them to equal the lamp lumens of the HPS. The life of the metal halide is about two and one haft times the life fluorescent lamps. Also, check the photo metrics of the t5 lamp with fixture- it may not have enough throw for that height.

Metal Halide is more efficient and has a longer lamp life than T-5's but it's color rendering is not very good if that is important.  One other advantage to fluorescent is the instant restrike capability.  You have to wait a while for the metal halide to get back to full birghtness once extinguished.  Instead of T-5's I would consider using biax lamps.  These lamps are less than two feet long and there are a lot of fixtures on the market that use this lamp in a fixture as an alternative to high bays.  The fixture is usually a 2'x2' package that can use 2 to 6 biax lamps which allows you to approach the lumen output of a metal halide high bay.  Another neat advantage with this type of lamp is the ability to control light levels by switching off lamps.  The only disadvantage I see is that you will be changing lamps more often because of a shorter lamp life and more lamps.

Before making this decision, most reputable manufactures repersentatives or distributers will do a free photometric analysis of your area.  You provide the room charastics (dimensions, colors, obstructions)and they will tell you how much light (in footcandles) you will achieve at floor level.  Have it done with MH and T5 fixtures.  Weigh your options; instant on, lamp life, total KWH, etc.  Color rendering is probably not a concern as pulse start MH is CRI:65-75, T5 CRI:85, but your current HPS is CRI:22 (CRI:100 is best (halogen/incandescent))

From a height of 35-40 feet the lighting requirements (fc) can not be all that high if you are currently using 250 hps. However if replacing one for one and maintain whatever levels are currently present 200 pulse start will not cut it. Mean lumens of a 250 hps is 26100 while 200 pulse start is around 16800. For a one to one replacement your looking at using something around 320w pulse start at 26400 mean lumens. As HPs is the most effiecent light source out there except for LED it will almost always require more power to trade for your color rendering. As for thinking about using T5 you would be crazy at that height.  Do not even think about it!

I agree, MH would seem to be the way to go in this application.  MH would definitely have a higher efficacy.

Regarding the color rendering mentioned by hbendillo, there are newer ceramic metal halides out there, I believe they have a better CRI than most fluorescents.