1 down jacket or rain jacket and vest?

If those are the only choices, then definitely the rain jacket and vest because I don't trust any light down jacket to provide adequate rain protection and I wouldn't want to wear something that warm while hiking anyhow.

It kind of depends on where you operate. Some areas along the coast get lots of rain. Some high altitude areas get very cold.

For both problems, I carry three items: one Mont-Bell down inner jacket, one Western Mountaineering down vest, and one AntiGravity Gear hooded rain jacket. That whole assembly is less than 18 ounces.

–B.G.–

Seperate rain jacket because if hiking and it's raining you don't want added insulation or else you'll sweat

I prefer vest because the torso skin temperature is highest so the insulation does the most good. An ounce of insulation reduces more heat loss

For summer a down vest paired with a wind shirt or light rain jacket like the GoLite Tumalo and you have a very versatile setup. The rain jacket/wind shirt over a down vest will keep you comfortable in wet windy alpine conditions in the summer. For shoulder seasons you will likely need something more substantial on the insulation side.

Think of a WPB parka as a "safety item" and the vest as a dual purpose item for both outdoor insulation and sleeping insulation. I've worn my vest in 24 F. temps in my 30 F. bag to stay warm. It worked.

A WPB or even just a plain waterproof non-breathable rain parka is an absolute must for backpacking anywhere outside the desert southwest. I live in Nevada's Mojave Desert and when I'm in the mountains above 5,000 ft. I always carry my parka. "Mountains make their own weather.", as the adage goes.

Without a rain parka (not a hoodless jacket) you are potentially a hypothermia victim. Even in 50 F. or 60 F. weather rain can soak the heat out of you and the slightest breeze will make the heat loss accelerate. I've seen it happen in Pennsylania in late September.

When you begin to do high altitude backpacking (above 5,000 ft.) you will want a light down jacket like the Eddie Bauer one I'm wearing in my avatar. Worn alone they are nice on chilly evenings. Worn under a WPB parka they are much warmer.

P.S. And they can lower the effective temperature rating of a sleeping bag by 10F. if you have enough room inside the bag to keep the down in both from being compressed too much.

Montbell UL with the pockets, no rain 50 degrees
Same with rain jacket, 35-50 degrees
Rain in the 60s, rain jacket only.
The above assumes in camp, not moving. When moving, my rain jacket alone is good to 50 degrees in the rain.
To me the heat I loose in my extremities is too great for a vest only warmth option, so if it's going to be 2 external wear items, i recommend down sleeves.

The nanopuff you mentioned is a good synthetic piece, and can be easily found on sale. Down is warmer for the weight, but doesn't handle moisture as well as synthetic.
For down, something like the Montbell UL inner Parka is a good bet, or better yet (but expensive) the Western Mountaineering hooded flash jacket.
I'd look at the Outdoor Research Helium II as a rain jacket. Its one of the lightest waterproof's out there and people seem to like it quite a bit.
If you are just getting in to backpacking, I'd carry more than a vest. Its a fairly specialized piece that tends to work well for summer but not so much for the other seasons, at least for me.

Have fun!

Just came across this jacket on sale as well. Nothing says UL, like a hideous shiny purple button down hooded anarak! But, seriously for the weight there isn't a whole lot that compares and the price is good.

http://www.departmentofgoods.com/stoic-hadron-down-anorak-mens