1st - check your states regulations. you may not pass inspection with a single fil.
if signle is okay:
i personally like the way a single filament looks better. and i'd expect better bulb life (though that's not very important for 2 bucks a bulb).
BUT, i'd get a dual cuz it adds some visibility. heck, drivers don't see us already. take off two more running lights and there's even less for them to see.
my 2 cents.