In general, more wind is going to be harder on plants, and spots at the bottom of a hill are going to tend to be colder then at the top of the hill. So for the best protection, I would go for a sheltered spot at a higher point in your yard. That's only an estimation though, it's hard to predict exactly where the coldest air is going to flow, and you can end up with cold pockets in unexpected places sometimes. Planted right up against the house (as long as it's not exposed to strong winds) is also a very good place for borderline plants. I've found the best way to figure out microclimates is observation--there are some plants that I know are sort of borderline for me, and if I have that same plant in multiple places in the yard then I can tell where it does the best and suffers the least frost damage.

I am trying to do the same thing and I read this in a gardening magazine. this fall buy a bag of daffodils and plant them around the yard in several places you think might be warmer. In the spring the daffodils that bloom first are in a warmer area.

The magazine told of someone who had done this and found that and area in her yard had blooms almost 3 weeks before some other parts.

If you ever get snow or ice, look and see where it melts first. Those are your warmest microclimates.