If you demo'd the S3 and liked it, pull the trigger. There's good reasons why it is such a popular ski up at Whistler.
If you are still shopping, why have you settled between the S3 and Influence 105? Those two skis have almost nothing in common, aside from both being skis. Different feel, different target skier, different performance band. That's why you'll be unlikely to find a head-to-head comparison of the two. . . unless you ask for it on a place like this board.
FWIW (and that isn't much), I like the Influence 105 a lot - I demo'd it twice last season - and I think that it is an underrated, under-loved ski. But it is nothing like a S3. The Influence is a damp, powerful ski, with the metal matrix under the topsheet and sidewall underfoot. The reason that I like the ski a lot is that it had a similar feel to my P100s, but they improved it in two ways - way more damp and solid on hard pack, combined with a more loose, modern feel in soft snow (probably from the rocker and being a bit more forward mounted than the P100). But those are exactly the reasons why it isn't a good comparable with the S3. To me, the Influence is more of a big mountain charger, solid on firm snow with soft snow capability. It kind of splits the difference between the Cochise and the Bonafide, but it is more in that category of stiff, burly skis (that might also include the Mantra, E98, Atomic Ritual, Cham 107). The P98 is the Line that is probably closer to the S3 in capability, although relatively more damp than the S3, it might give you more firm snow grip for groomers without totally losing the fun-factor of the S3 that you probably enjoyed in the powder at Whistler.
The S3, on the other hand, is a softer flexing, more forgiving, more snappy wood core ski that is respectable on groomers (up to a limit) but biased toward soft snow. Comparable skis would be: Atomic Access (if you are a value operator, the best deal on the ski wall in my opinion), Armada TST, Cham 97 (metal but 5 point shape aimed at soft snow), or maybe the Nordica Soul Rider (a really fun poppy twin).
This is where magazine/internet research can be dangerous. You seem to have concluded that the Influence 105 would be the better powder ski of the two - and the greater width might suggest that. But shape, flex and construction are just as important factors and as a result, the S3 is likely to outperform the Influence 105 as a pure powder ski (unless you want that heavier, more damp feel). If the S3 was solid enough for you on groomers, pick it up and don't look back.
Both skis, or types of skis, would be worthy choices for a OSQ to take out west. Neither is better nor more "expert." The question is what do you want? Powerful and damp? Light and poppy? Stable and cruisy? Fun and surfy? Do you want to optimize for high speed on groomers on the front side of Blackcomb, or for runs off-piste in soft snow off the peak chair? Only you know that. Your height/weight is in the middle. . . although "not into overly steep hardpack" suggests that a more playful, less charger ski is probably the direction that you would enjoy most.
One thing that both the S3 and the Influence 105 have in common. . . they are both being phased out. So you could probably pick up either for 1/2 price this summer. Or if you are willing to wait and want the latest and greatest, you might check out next year's Line Sick Day 98 or the Rossi Sin 7 (98 underfoot), both of which sound like evolutions of the S3 idea. . . at least from the press coverage.
Hope that helps.