i have had several of those old-style, 2-strap shimano spd sandals, a pair of nashbars, and a pair of keens. i have flat arches, so i can't give you any useful information for your arch issue.

i wear sandals for commuting and touring, and of the 3 models above, i prefer the old 2-strap shimanos. they aren't light or stiff, but they are built tough and last. the nashbars are alright but the adjustable heel didn't agree with me. i am not keen (haha) on the keens because i have some chafing and irritation from their straps and i don't care for the closed toe. they do not drain pebbles, water, etc well and i feel like i might as well wear my sidis. the keens do seem a tad lighter and stiffer, and they give my feet some cool, striped tanlines, but i wouldn't buy them again. and they are sorta pricey.

i guess this probably doesn't help you much. i know rei carries the keens and has a fairly generous return policy. maybe try some from rei, return them, and buy them from someplace cheap if you like them.