the sun t5 is ok, you can also go with a GE i think it's rated at 6500K.the GE provides the most par out of all the bulbs.or you could just switch out one of the pure actinic bulbs with an aquablue.
it really all depends on what color you would like for your tank.
hey janey, what brand fixture is it? It sounds like a teklight from description of the switch setup.
The aqua blues are about 11K so depending on what type of corals you are keeping (softies, lps, sps, etc.) that may be enough. On my tank i have 3 aqua blues and 3 blue plus - All ATI and i'm easily keeping SPS so you might want to consider replacing one of the actinics with another aqua blue.
If you do want to get a 6500K (daylight) bulb for some extra intensity (more yellow color bulb but it might not be that bad if you just replace one of the aqua blues and keep the rest of the blue plus and actinics), get the GE daylight 6500k bulb. It has the best PAR and lux ratings out of pretty much any other T-5 HO bulb.