Each system would require the purchase of appropriate ballasts, bulbs, endcaps, standoff mounts, reflectors.
Here's an example of a T5 HO retrofit kit, which includes all the parts needed to install the lights into a canopy (notice the price includes the bulbs):
http://www.reefgeek.com/lighting/T5...High-Output_Retrofit_Kit_w!_Bulbs_by_ReefGeek
If you shop around and wait for things to go on sale, you can piece together your own DIY parts for less money. But, that might mean a lot of internet and / or local searching, as well as waiting a couple of months for things to go on sale.
The cost difference for ballasts is probably the biggest difference, however, if you're forced to set up twice as many 2' T8 bulbs compared to fewer 4' T5 HO bulbs, purchasing 2 T5 HO ballasts should be no more or less expensive than buying 4 T8 ballasts.
2 x 54w (4') T5 HO ballast:
http://www.ballastwise.com/item.asp?PID=28&FID=15&level=1
Endcaps and standoff mounts cost pretty much the same for either system...
http://www.drsfostersmith.com/produ...&Ntx=mode+matchallpartial&Np=1&pc=1&N=0&Nty=1
T5 HO bulbs run about $20 a piece for 4' bulbs, while 2' T8 bulbs run $12 - $15. Right there alone, extra money would be spent on T8 bulbs initially and at each replacement than T5 HO bulbs.
T5 HO lighting has the option (and a highly recommended option) for an individual reflector for each bulb, which is said to increase light output to the tank by 200% - 300%
http://www.reefgeek.com/lighting/T5...5_Retrofit_Reflector_for_1-54W_Bulb_by_IceCap
http://www.reefgeek.com/lighting/T5...t_Reflector_for_1-54W_Bulb_by_Sunlight_Supply
http://www.reefgeek.com/lighting/T5...t_Reflector_for_1-54W_Bulb_by_Sunlight_Supply
When it all comes down to it, if you're going to buy all the pieces (be it separately or in a retrofit kit), you'll get more lighting bang for your buck if you choose T5 HO over T8. Plus, there are dozens of bulb choices available for T5 HO, although there certainly is not a lack of T8 bulb choices.