Ultimately, it depends on your personal preference and presentation.
One of the primary reasons that magicians have used dollar and half-dollar coins is the size. They are big and easy to see, and the larger the coin, the more "impossible" some routines will seem ("How can he be hiding such a big coin...?"). If that matters to you, than go for the biggest coins you can find.
But... you should think about how you are going to justify the use of coins which are uncommon or out of circulation. With most stage coinwork, it really doesn't matter, as the coins are never really seen clearly anyway. They're just shiny discs of metal with which the magician is doing amazing things; some performers don't even use real coins for stage work but, instead, use palming/production coins which are optimized for magic (milling, thickness, etc.) but don't really resemble any "real" coin.
For close-up work, though, it's not necessarily as easy to get away with using "odd" coins unless you have a presentation that justifies it. It's not difficult at all to come up with presentations that explain the use of old coins. As a simple example, check out Ron Bauer's "Gadabout Coins Revisited." Also, if you are worried about your audience being suspicious of the coins, let them examine a few of them (it's not hard to develop a routine that allows for audience examination of the coins while concealing the gimmicks... depending, in part, on the gimmick).
I use in-circulation Canadian coins for most of my coin work. Canadian dollar and two-dollar coins aren't as big as US halves, but they are big enough, and they look interesting. I have a lot of US halves and English pennies and a few sets of standard gimmicks made from those coins. I don't work with them a lot, but maybe some day... or maybe I'll just buy gimmicks made from Canadian coins. It just hasn't been a big issue for me yet because most of the coin magic I do doesn't require gimmicked coins.
I'm in kind of a similar situation in that I have an infatuation with the Okito Box. These things just don't exist outside of magic presentations, and some guys won't use them because they are not ordinary, everyday objects. Although I can see the relevance and use for ordinary object theory, I feel that it sometimes gets overemphasized. I have no problem with David Roth's solution: he makes no effort to suggest that the Okito Box is an ordinary object and doesn't try to pretend it's an old pill box... he introduces it as a magic box and leaves it at that.
Sometimes we get so caught up in the idea of doing magic with ordinary objects that we lose sight of the fact that magic is meant to be extradordinary... so why can't our props be, as well?