I thought that I should mix up this topic a bit, as a supporter of the 45° :-) I have an INON 45° Viewfinder installed in my Housing.
I would generally say that it does not matter which type of Viewfinder that you have, as long as you have one.
For me there are 2 clear areas of advantage/disadvantage:
* Low down/Macro: An angled has the clear advantage. You can stay a bit further away from the bottom => less sediment stirred up.
* Half-half: Here the Straight has its clear advantage over the angled.
For the rest you simply automatically adjust to it. It took me perhaps 5 dives or so, until my brain had synched to it.
I, personally, do not find it troublesome to follow fish, mantas with the 45° Viewfinder.
/Erik
Ps! Oddly enough it seems like Europe prefers 45° Viewfinders and US straight Viewfinders.