I wouldn't give too much thought to the weight difference on the website personally.
IMO Rocky's alloy bikes are ugly and their carbon bikes are gorgeous. So I'd push for carbon based on that alone

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I personally would never get the 70 over the 50. I really don't see $1600 of value between the two. You still need to take both model's pads and rotors and put them in the garbage before you ride. You get the same hubs. The rims
really aren't actually an upgrade. Likewise handlebars. Sure the brakes and brake levers are better on the 70, but once you put the proper rotors and pads on it's hard to tell the difference except there's no free stroke adjustment on the SLX (which does almost nothing on shimano systems anyway except allowing you to get slightly better bleeds) and you don't get the fancy dimples on the levers of the SLX. You can swap to an XT shifter at some point for minimal cost. Personally I'd go straight to XTR (on both models) because the shiftier is the biggest difference in the actual feel of the system. The actual big difference is the fork.... but I don't see that being worth the cash to upgrade. I can't think of a single person who actually spent time dialing in the Yari who wanted to upgrade it to the charger 2 system after giving the Yari a proper service and seal upgrade (which runs less than the 300 dollar charger system). Even if you DO want a lyrik ultimate, the upgrade cost runs less than 1600 bucks.
So if you want a 70, do yourself a favor and spend the extra 1600 bucks (or less) doing the upgrades that actually matter