Are we talking MH here?
If you're using supplemental actinics than for the MH,I would go with 10k.Like Biff said,you get better growth with the 10k.
No supplemental acticnic then 14k.Personally 20k is too blue for my taste,the choice is up to you.
I would go with UV Super Actinic and ATI Blue Plus as supplements if you decide to go T5.