Last shot, unfortunately, I cannot answer you on which will go the farthest, as I do not have either at the moment, and my time playing with them was not in a circumstance where I could wring them out. Hope to have the opportunity to in the coming month, but not yet. I can tell you that my 1600 has consistently hit the leafy trees at near 1700 yards in good conditions, and that was hand held, no tripod mount. Longest I hit animals was 1250, these were cows. I have hit reflective targets at over 1700 handheld using scan, I suspect that attached to a tripod, it would go a mite bit further.
From the folks that posted here on another thread that have the 2700 already, at least one person is hitting trees to 3k. Leica has historically been very good about the distances they claim, pretty consistently exceeding what they promise, so this does not surprise me, especially considering the performance of my 1600.
As I remember, Sig claims 1400 on deer, 1800 on trees, and 2k on reflective steel. You probably will exceed that mightily if you are ranging something like a stop sign or maybe a cliff or whatever. I hear you can hit about 3k or maybe more under ideal conditions with it as well, but again, as I don't have one, I can't say for sure. But based on what I hear, I'd say they're probably about equal, with conditions and individual unit variation probably being more significant than anything else between them in terms of distance.
However, what sets the Sig apart is it's integrated ballistic capabilities, otherwise it's basically a slightly more powerful 2200. Sig themselves say it only gets you another 5% in ranging distance over the 2200. So you are paying an additional 1200 or so over the 2200 for the ballistic stuff, that is what makes it special, and that is also what makes it's MSRP almost twice what the Leica's is.
If that is not something that appeals to you and we therefore remove that from the table of consideration, and we also assume ranging power is equal, which is I think pretty safe to assume, the Sig cycles and 're-ranges' a lot faster than the Leica (should you miss your target or whatever), but the Leica has a lot nicer glass to see things that are super long range and in low light, no doubt. The Sig guys themselves agree with that disparity. But most importantly at those long ranges, the beam divergence is basically half of what the sig's is. The sig is 1.3x1.3 mils, where the Leica is .5x1.2. IIRC, it is a horizontal box so you are less likely to hit things in front or behind, but not sure on that though but that's what I remember. At long range, I think that smaller divergence really matters, especially when trying to hit a deer, not the ground in front of it, or the tree 100 yards behind it.
Again, I am not anywhere close to done evaluating these, so I don't want to steer you in any particular direction in case I mislead you, but if you don't care about the ballistics, I don't think the 2400 makes sense in that case, that's what you are paying a large premium for with that unit. If you don't plan to use those capabilities, you can save the coin and get the Leica and the benefits it has in other areas.
Hope that helps!