first off, PC's are just fine for most nanos without high light corals
i don't think they make T5's that will fit your tank 24" are the shortest made by the major companys right now
T5 is just a very thin flourecent tube 5/4" - their major pros are the ability to mix and match spectrums to get exactly what you want, life span (some say 2 years), and energy consumption to output ratio. cons a little expensive for the watt/dollar ratio and the cheapy versions don't offer the same benifits as the high end sets - like individual reflectors and HO wattage
MH best watt/dollar ratio if you want intense light which some corals like acros apreciate, cons - not many bulb choices if your looking at 70W's but with 150Wers and up you get a lot more choices but then need to deal with the heat issues in a nano. also most bulbs need some kind of suplimental lighting anyway
bottom line long skinny tank T5's are a very nice option, square or deep tank then MH is better, but in the end its your coral choice and tank theme that makes the real difference - what kind of reef are you building??? that is what chooses your lighting for you
