The most breathable rain jacket material is Event. The second most breathable is Propore (Dry Ducks, O2, etc.). Most windshirts are more breathable than both, but they aren't waterproof. Event is very expensive, while Propore jackets are very cheap. So, I would either get:
1) Propore rain jacket, no windshirt
2) Propore rain jacket, cheap windshirt
3) Cheap poncho, cheap windshirt
Now, just to throw something else into this mix: some rain jackets have pit zips (or other vents). Jackets with vents, can breath better than the most breathable materials. I don't think any of the Propore jackets have vents. So, that leads to the following option:
4) Cheap rain jacket with vents, no windshirt
5) Cheap rain jacket with vents, cheap windshirt
In order of weight, I think it is 1,3,2,4,5 (with a significant jump for number 4 and 5). Personally, I would go with either option 1 or 2.