Your T5 options listed are all very similar... sorry I cannot offer much opinion on the different brands, but that 4 x 24 configuration should work out great for you. I wouldn't do the 30" fixture on a tank that is 24" wide... only because I think it looks a bit sloppy for the light to spill out, if that makes sense.

A 250w MH seems like overkill for a 20g and will lead to heat, evaporation issues. I think you'd be happier with any of those T5 options.