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1/2 or 7/16 doesnt matter really, they both fit on 1/2 inch barbs. R-3603 tygon is very soft and has to have a minimum bend radius, which cramped systems cause problems with.
I use 7/16 masterkleer, while it is a pain to get on 1/2" barbs, it provides a slightly better seal. This stuff seems to bend pretty tightly before it kinks, so decide based on that. If you already have an apogee and you are short on money, stay put. The newer blocks dont offer up *too* much more cooling power, but I'll leave that consideration up for others to comment on, I'm not 100% up to date. (I know the new dtek fuzion is supposed to be quite good, but I dont know how much better it is now)