Sorry to be the bearer of bad news, but really you need both.
A waterproof shell is designed to keep the rain and wind out.
An insulated jacket is designed to keep the heat in.
Whilst the two are linked, they do different jobs.
A traditional layering system consists of baselayer->midlayer (fleece or softshell most commonly->waterproof outer shell, with an insulating layer in the rucksack.
The waterproof shell is fairly obvious - keep the water out. Features to look out for include a good wired peak to keep the hood in shape and some of the wind off your face in wind and something long enough to cover your backside (if you find it comfortable.)
An insulated layer is generally for rest stops - something you can chuck on as when you stop walking you stop generating heat, and you want something to trap it. It can also be useful for the easier stretches where you aren't generating as much heat.
Insulation wise, you've got either down or synthetics. Down is more effective but when wetted out doesn't offer much insulation, though many modern variants offer hydrophobic down which resists water more effectively and offers some insulation when soaked. Synthetics are less warm per weight/heavier but offer more insulation when soaked, even than hydrophobic down. Personally I've always managed fine with a down jacket underneath a waterproof as I feel the cold and appreciate the better warmth for weight ratio, but both options have their merits.
Now just to confuse things, many manufacturers now offer insulated jackets for on the move - synthetic jackets with less filling than a traditional rest stop jacket that are much more breathable, aimed at people who are on the go in cold climates. These replace fleece as a midlayer, and offer enough water resistance to mean that the waterproof shell can stay in the rucksack unless the rain gets very heavy - and when it does, it can be chucked over the top.
Still with me?
In my opinion, you need a good waterproof shell regardless. Now there are 'winter,' orientated shells, and lighter weight 'summer,' shells. The summer ones tending to be lightweight as they'll spend more time in a rucksack. So I'd start with something more heavy duty. Get down to an outdoors shop and try a few on - I can chuck out names like the Montane Direct Ascent, Rab Latok etc but fit is important. Remember to factor in a bit of room for other layers underneath.
Worthy read:
http://www.walkhighlands.co.uk/news/winter-waterproof-jackets-grouptest/0011846/And some sort of insulating jacket is going to be important. Whether you want to go down or synthetic is your call really, I don't think there's a right or wrong answer. And there's loads to choose from. This may be a handy read:
http://www.walkhighlands.co.uk/news/winter-insulated-jackets/0012184/Personally I think the 'in between,' products - lightweight insulating synthetics for on the go, windproofs, Vapour Rise-type midlayers are all very nice things to have, but should come once you've got your waterproof and rest stop layers. And a bit of time on the hill to work out how it all fits together and what will fit your needs best.