Actually, the T5 uses more space, sort of. The T5 with a reflector is going to take up 2 - 3", depending on refector. A VHO T12 is 1 1/2", the reflector is internal on the most commonly used VHO's. Most dont use external reflectors. But in reality, they take the same space because the end cap mounts are the same width at about 2.5". If you float the endcaps the VHO actually will use less space. If you dont use a reflector with the T5, you may as well use VHO, you'd be better off.
Arc Eye ... there are lumen / watt and other quantitative comparisons of VHO and T5. Nobody seems to be able to provide specific data of reflector effectiveness however. All I can go by is my personal experience with both T5 and VHO and T5 with individual reflectors and with one large reflector. Thats personal experience with observation too, not even instruments.
I built my light rack to accomodate either VHO or T5. The only reason why I would use the VHO option was for the UVL super actinic lamps. I never was impressed much by the T5 super actinic. UVL's T5 super actinic is a signifigant step up in output compared to a cheap super actinic T5 like Current USA's chinese made lamps for example. But still not really impressed by it. I have to say though, on the IceCap ballast which I just switched to, the UVL actinic T5 is not too shabby. First time I have seen a super actinic T5 atleast approaching that of VHO. The color is good and the output is respectable. With the reflector and the IceCap ballast, honestly speaking, I dont think that VHO super actinic is going to put more light into the tank than the overdriven UVL T5 super actinic. Color may be a bit better with the VHO actinic but I dont think it will compete in output, again, into the tank. I thought it may be close with the T5 at 54 watts. I thought that with the color edge, it may be worthwhile to use VHO over T5 Super actinic at 54 watts. I may be wrong, the T5 may have signifigantly more output making it into the tank at 54 watts with the reflector. But with a visual comparison, I thought it may be worth trying the VHO actinic over the T5. But at 85 or 80 watts, I dont think the VHO can compete in regard to light in the tank. I think the color is close enough to make it not worth losing the output advantage that I am condident that the overdriven T5 super actinic will have.
For what it's worth and I'm not invested either way.