Bill,
I chose to use this aluminum one from Summit, $50.00....It is nicely made, works great, priced right, and looks very good.... I don't see the necessity of having a coolant overflow finned, other than if you like the appearance, yes it might cool the accumulated coolant slightly quicker, but I would think that would be a negligable amount, and not affect vehicle performance at all.
Hope this helps...
SUM-300101
Coolant Overflow
Here is a pic (at least of the top) so you can reference the size..
J.T.