Of the two Symmars, I would keep the 5.6 just to have that little bit of extra light when focusing. If this is for school use, then it will also help keep students from getting confused when they see an f-stop that is not on the normal scale. As for which lens to get for portraits, I think the 24 inch is a good length for this reason: The 450mm is "normal" for 11x14, giving the same perspective as 300mm on 8x10. That being said, if the 450mm gives you the portrait perspective you want on 8x10, then multiply that by 1.5 to get the same relative perspective, which is 675mm on 11x14. Close enough to 24 inches to work. The other option would be a 480mm lens on 11x14 which gives as close to a 420mm on 8x10 perspective as you can get.