I never really understood why people use a bivy sack with a tarp. My solution is simple: just set the tarp up well!
If you're not confident on your tarp setup skills, get a bigger tarp! For example, Gossamer Gear's two person tarp weighs a bit more than a one person tarp, but it gives you significantly more space. It's worth the slightly extra weight to get a ton of breathing room (and it will make up for any sloppy setups).
For a good setup, start with a good location. Find a place that is sheltered by rocks or trees. Favor low elevation locations over the high ones. Just doing that will solve most of your problems.
In a windy rainstorm, just pitch the tarp flush to the ground on the side the wind is coming from. The other sides shouldn't be that high off the ground either. You'll have to crawl and squeeze to get in it. Because the wind is fierce, you won't get condensation (unless you're touching the tarp with your sleeping bag, so make sure that you have a little clearance).
The only time I would consider bringing a bivy is when you're camping during a snowstorm. Snow has a devilish way of blowing into a tarp, defying gravity.
Otherwise, I would stick with a tarp and get a big one until your tarp setting up skills improve.