Bontrager Packable Stormshell - Gregg's Cycles
This is what I have. Really light, packs up into a square about the size of a rear jersey pocket and decently warm. I like the yellow over the black just because its different but color is up to you. Great jacket overall. I also have the cannondale Morphis. Its a good amount warmer and breaks into a vest, and is pretty water resistant, but its a little heavy for 70º even in vest form

Rapha Rain Jacket -end thread-

It really depends on how long you are planning to ride. Anything longer than an hour in a steady rain, you will get wet, either inside or outside, regardless what you wear; on top of that, there are other parts of your body will also get web, e.g. shoes/socks. The key question is warmth.