I’ve spent far too much time in the last year comparing XC geometry and playing around with my own setup, and I’m starting to come to a few conclusions as to what works for me. 
I’ve decided that for fast XC riding and racing, I really prefer to ride with only as much suspension as I really need for the given terrain. Having more suspension/bike than you need will likely slow you down and can actually take some of the excitement out of riding.
In my opinion, having more suspension travel is only better if you’re riding more serious features, and taking bigger hits out on the trail. If you look at modern XC courses up close, the stuff they’re riding is actually pretty serious. They make it look smooth and easy on TV, but those features are tough! If you’re riding trails like that, a bike with 120mm of travel front and rear is probably a good idea. If the terrain you’re riding is quite a bit more mellow (flow with some rocks/roots) then you might not need so much bike.
Ultimately, it does depend on the terrain you’re riding and your skill level. Highly skilled athletes can ride the rough stuff with very little travel, but for most of us that would probably be a bit scary and also hard on our bodies.
Bikes that shine on climbs are typically stiffer which makes them less forgiving on the descent. A hardtail is probably going to be the quickest on smoother climbs, but if things get rough, a little suspension can help you keep the power down. This is where some of the “World Cup” style bikes start to come into play (Trek Supercaliber, Specialized Epic WC, the upcoming BH that was seen in Paris this week).
In my opinion, these are great options for those courses that are a little more tame or those who are really skilled and can handle a bike really well in tougher conditions. I’m hoping that before long, some of these can be made with standard shocks (I think they’re all proprietary at this point ?) which will make long-term serviceability a bit easier down the road. These all fit two big bottles in the main triangle though and can make great long-haul bikes and solid do-it-all all rigs. I’d say for a lot of people they would double as gravel/adventure bikes really well too!
Next up you’ve got your “full suspension pure race bikes” such as the Canyon Lux, Pinarello XC, Cervelo ZFS-5, Cannondale Scalpel, Santa Cruz Blur XC, etc.) These are typically around 100mm travel front and rear. These bikes are really solid for people who want a really fast, light, and responsive bike that’s also pretty capable on most XC terrain. You’re not giving up much in weight, but you’re gaining quite a bit of traction and support. The geometry is still designed to be really quick handling, but the extra few mm of rear travel helps to smooth things out. With standard shocks, these are going to be a bit easier to service and find parts for down the road as well. These can be great all-arounder MTBs in the right places!
Finally, you’ve got your more “modern” XC bikes or “down-country” bikes which start to blur the line between XC and lightweight, short-travel trail bikes. These are usually 120mm of travel front and rear and also sport the most capable geometry as well. Their HTAs are pretty slack and definitely don’t handle as sharply as the other bikes, but are really capable on the descents. If you’re riding a lot of really rough terrain, aren’t as confident on the trails you’re riding, or spend a lot of time in the saddle, these bikes are great choices. A lot of marathon racers use these types of bikes as they’re simply more capable and forgiving over the long haul, and many XCO racers are opting for them on the rowdier World Cup courses as they make the technical sections more manageable and allow for more recovery on the descents. These would be your Specialized Epic 8s, Scott Sparks, Canyon Lux Trails, Santa Cruz Blur TRs, Allied BC40s, etc.
Again, any and all of these bikes are incredible, but the right one for you really depends on:
- The terrain you’re going to be riding.
- Your bike handling skills.
- Your priorities in terms of climbing vs. descending, efficiency vs. comfort, etc.
Since you already have a pretty capable trail bike that looks to fall into more of the down-country category, maybe you should pick something up that’s a bit more responsive. 
Another thing to consider is that many of the bikes today are rated for multiple different suspension travel options which could make one bike able to do most of what you need. Santa Cruz does a good job showing what their Blur looks like in terms of geometry with two different travel options (XC vs. TR) which is really handy. Even having a second fork to swap in could make a big difference in terms of versatility!
I obviously haven’t tried a 120mm bike though so maybe I’m way off base.