The ultimate XC bike. This is going to be something that is purely subjective to the user. But I can give you my imput based on 15 years of racing XC and endurance events. From local events to the Leadville 100 to a 3rd place finish at the 24-hour global championships.
First off... you mentioned similar components for both builds. What you listed sounds somewhat acceptable for a hardtail build, but the I would argue that the SID is too little fork for the Mojo. At 100mm I think you might find it a little unbalanced for the 5.5 inches in the rear, plus it may steepen steering beyond a comfortable level- for you.
If weight is your ultimate concern, go for the Tranny, or even a lightweight carbon option from another manufacturer. Period. The Tranny will always be lighter than the mojo. This doesn't always transfer into being faster.
Let me repeat that:
Lightweight doesn't always mean faster. It definitely helps, but trumping performance with lightweight typically results in an uncomfortable bike.
So the real question is where are you planning on deploying this ultimate XC machine. The Ultimate XC Machine for singletracks of places like Wisconsin (go badgers) or Texas might be a singlespeed 29er. If you are planning on racing XC, typically XC courses are relatvely smooth, non-technical affairs and your uber-light build and your big ring might be all you need. In such cases, 5.5 inches of suspension is probably overkill. Assuming all you are doing is racing.
I consider myself an XC/All mountain rider. I need a bike that is lightweight, strong, efficient, easy to control, and comfortable. For where I live the Mojo fits that build to a tee. You mentioned climbing. Here, I feel that my Mojo is a superior climber to any hardtail I have ever ridden, but I ride a lot of techy, loose climbs. If you're in pure fireroad country.. this might not be for you. But I think you will find that the notion that full suspension bikes are poor climbers is a severely antiquated one, especially as applied to the Mojo.
I have raced my Mojo XC. I have done all-days epics with 10,000ft of vertical up and down. I have employed it in multi-day shuttle runs. I like my bike in any terrain I feel comfortable riding, and I think that range has increased since I got my mojo.
I would like to advise you to get a mojo. I think that in terms of all-around performance, comfort, and light weight there is no better bike out there. If you really enjoy riding, if it is something that you love, than you will love the mojo. If you love competition, love racing in a pure form, then you might prefer a HT.
So... unless you're winning races for a living. Unless your babies will go hungry and without shoes unless you stand tall on that podium, you will probably have more fun on a mojo.
Just be honest with yourself about where and what you like to ride and your decision should be easy.
