It's a day hike. Take them both - the worst case scenario is that it gets late (or you're lost) and it starts raining, snowing, sleeting, or all three. If you have to spend the night out, having both, instead of just one, might just mean the difference between spending a comfortable night and a close call - that could go either way.

The problem with bivy sacks isn't whether they will turn the weather. It's the fact that, at some point, you need to get in or out of the bivy sack - at which point, without a tarp, water gets in.

I'd take both. But that's just me.