What is it that your wanting from the S100 that you can't get? Let me assure that wanderlust your feeling never goes away. There is always that next level especially when long glass comes into play.
To me it comes down to this. If you want to shoot wildlife with big zooms and do it "relatively" cheaply then Canon is the best choice. Personally I would skip the entry level and go straight for a used prosumer body from a reputable company (B&H, Adorama, KEH,) or from POTN. The 7D is a great camera that covers most shooting situations and can be had for a good price used. Full frame is the next level but I don't think you want to go there based on the availability of used bodies and the price of the new ones.
I can't speak alot for Nikon except to say they are very low noise and a great system. They seem to be the choice of pro shooters who shoot mostly indoors at higher ISO. That's not to say Canon is a slouch at high ISO. Currently in the newest bodies they have closed the gap maybe even taken the lead by using less MPs.
Next suggestion would be a 4/3's system. I've seen some amazing photos from these little guys. They offer up to 10 fps, super low noise, and some amazing glass. I think sony has Zeiss lenses for theirs. The major drawback IMO is reach. If you want to shoot birds with a 400mm your outa luck. Otherwise I think they would make a great system to enable a casual user to take professional images.
One last note. a 10D canon can be had for about $100 and if paired with a great lens it'll make wonderful images. A 1Dx Mark IV is about $7000 and if you put a crappy lens on it it'll take crappy images. Find a good middle of the road body and buy high end glass. Glass usually holds or increases in value, bodies drop like a rock.
Also, Photoshop lightroom is a must if you are serious about your images.
Lots to consider give me a shout if you want and I'll offer advice to help you avoid the mistakes I made.