I'd say go wth the half dollars.
I'm quite unsure why you Americans hate your 50p pieces

but alas it seems that no one has them on them! I asked loads of Americans who come into my workplace if they had any spare I could buy off them and the general answer was "Half dollars? Oooh boy I aint used one of them in a looooong time..."
Either way... I use half dollars to practice with. yeah they look out of place if you perform with them, but you can hand it out for nspection anyway. Furthermore, once you have the moves down with a half dollar, it's much easier to be able to use a normal smaller coin.
I started coin rolling with half dollars, and can now coin roll a 10p (much much smaller). I started palming with half dollars, and can now palm a £1 coin (about half the size)
So yeah. Be a bohemian. Carry half dollars on you.
Just don't spend 'em

Practice hard with them instead. Then, when you got the moves down, you cna be an impromptu wizard with just a handful of change and a deck of cards.
Furthermore, I recommended On The Spot with Greg Wilson. There's several effects in there that use coins, pocket change etc and some great other bits too!
You wont find much better anywhere and it's nothing - a rigmarole with a few bits of paper and lots of spiel. That is Mentalism
Tony Corinda