Good thread! I have often encountered the same "what type of 2-ski quiver should I get" question in my own skiing. Here is what I came up with, pros and cons of each. Keep in mind that I have skied a ton of skis this year, pretty much every big name ski, and most, but not all, fall somewhere into one of these categories. Below are generalizations, but tend to be true much of the time.
78mm ski vs. mid 80's ski for frontside use:
78mm ski Pros: great edge hold, more responsive on firm snow, closer to race-ski feel that is quite addicting if you are skiing hard snow and groomers. A great width for people who need a responsive ski for bumps and don't need much float (a lot of East Coast skiers pick up 78mm skis, and so do West Coast skiers as part of a 2-ski quiver).
78mm ski cons: a little narrow for soft snow (not an issue if you have a wider ski), many are pretty stiff in bumps (the Peak 78 is an exception, as it is exceptional off-piste, provided you don't need much floatation)
85mm pros: sometimes more forgiving, as they seem to be built with off-piste skiing in mind (although there are exceptions on both sides), typically a bit better in soft snow, and typically a bigger turn radius, which could be a plus or minus depending on your outlook.
85mm cons: edgehold lacking compared to 78mm, slower on edge, typically not as powerful feeling
95mm vs. 105-115mm skis for new snow:
95mm Pros: still decent on firmer snow, most of these designs are typically "all-mountain" more than backside, still fairly responsive for mid-width ski. Probably a good width to own for just one ski.
95mm Cons: lack of float in softer, heavy snow can be a hinderance, depending on snow quality. In light snow, 95mm can be more than plenty of width. In heavy snow, it can prevent you from skiing to your potential
105-115mm pros: much better float; you are getting to that point where float will pretty much handle any snow condition at most any pitch angle. Bigger turn radius often perfect for big-turn Western skiing
105-115mm cons: starting to get away from all-day versatility, depending on the ski, the conditions, and the terrain. Some of these are great in end-of-day, skied-out snow, and some aren't. Also, they feel pretty wide, and don't work for everyone's skiing style. Keeping parallel shins is really key for skiing bigger, stiff skis, unless you are skiing rockered stuff, which responds to both good and bad input.
With regard to cambered and rockered stuff; that is probably a question that you will have to answer on your own. There isn't one "right" answer, as everyone has different expectations and different skiing styles. No one ski can do it all, so ignore anyone who tells you that. A ski purchase is always making some sort of compromise, unless you have 3 or 4 other skis in your quiver already.
Here is my current situation on a 2ski quiver:
Kastle MX78 176cm
Elan 1010 183cm
Kastle rips up groomers like a race ski, and is a true throughbred. You have to ski full-on to get the most out of this ski. Not the best crud ski: it is pretty stuff.
Elan is superb in 6-8 inches or more of new, and softer bumps. Lacking on firm snow, not very responsive, but pretty good for a ski of this width on groomers. Very versatile for what it is, and plenty of width for any condition I will encounter for my weight.
What is lacking out of those 2 skis is that softer, mid-width crud and bump ski for when the snow is soft, but we haven't had a whole lot of actual fresh snow. That is where that mid 80's to mid 90's ski comes in handy. It will rock the crud and be much easier to ski than the MX78, but much easier and more powerful on firmer snow than the 1010. I can make do without that mid-width ski, but would rather have one. OTOH, I don't want to give up the superb race-like performance of the MX78 when we don't have fresh snow and I want to rip big turns at speed, and the 1010 isn't going anywhere, unless a better wide ski comes along, which I have yet to find. Had I purchased the Peak 78, I would have lost that race-ski performance, but gained a more versatile ski, and probably covered the 2-ski quiver much better. But, the Peak 78, as great as it is, is no MX78 at speed.
My thoughts? Get the narrower ski you really want, and just assume you will ski the wider ski if much new snow is falling. That Peak 78 is as good as any narrower ski, and if you are getting something wider anyways, the extra width of something 85-90mm will be mostly wasted. The trick is finding a good 78mm ski that isn't basically a full-on carver, but that Peak 78 fits the bill as well as any ski around. Once you have that figured out, demo some wider skis, and see what works. Try something 95mm and 105-115mm underfoot, and see what you like the feel of. Again, no "right" answer to this one. For every customer I speak to who will not ski anything under 100mm, there is another who says "you know, I just can't warm up to really wide skis; I haven't found one I like over 90mm or so". Had a guy in today and thought the 1010 was "too wide, given we only had 10" overnight; great early in the day, but wishing I had a bit narrower ski in the PM" and purchased the 999 instead; it fit his skiing style better and was more of an everyday ski that made sense for him, as he owns a business, has a family, and only gets 15-20 days a year in. We all like to think we are locals, and will get to ski every big dump of the year, but in all honesty, most people are limited to weekends and perhaps 2 trips a year, which puts a different emphasis on things than if you are skiing 5 days a week, all winter long.
I like your idea of a Peak 78 for a frontside ski: this is as good as versatile frontside skis gets. If looking a bit wider, the Sultan 85 makes tons of sense, as does the Peak 88. The Noridca HR (the one that replaced the Afterburner) also is worth checking out, if you are trying to stay away from skis that are really stiff, and instead great all-mountain tools. With wide skis, there are so many choices, it is hard to know where to start, but some good ones: Watea 94, Elan 999 and 1010, Volkl Mantra, Blizzard Atlas, Dynastar Big or Huge, Blizzard Answer IQ, Blizzard Argos, Rossi S7, K2 Obsethed, Dynastar Legend Pro Rider, Kastle MX98....
A good analogy is bikes: some people ride singlespeeds, some ride fixies, some ride geared road bikes, some ride hardtails, some ride 5" trail mountain bikes, some like 29ers, some ride trails on cyclocross bikes, some like 7" freeride bikes, some like XC race bikes.....And some own a bike of each! I have about 6 bikes in my garage, but honestly, the ones that get the most love are the hardtail 29er, my road bike (obviously) and my road fixie, especially this time of year, when I don't want to deal with cleaning dirty drivetrains and want to work on my spin and power.