Sonic,
It is hard to make a choice because the options each have their strengths and weaknesses, so you really have to decide what you want, and what things you don't care about....and what weaknesses you can live with. From the ones I am most familiar with...the 2400 BDX, 2800.com and 2400 ABS (to a lesser degree), I would not tell you which to buy, because only you can know that. That said, here are some thoughts to consider:
From a pure ranging perspective, I like the 2800.com best. Of these choices, it has the smallest beam divergence, the smallest aiming point and the best aligned sensor. Leica requires that no part of their sensor go outside the reticle. Which means windage wise, they are perfect, though elevation wise, it could be centered, a little low, or a little high...but always contained within the reticle. The same cannot be said for Sigs, where their tolerance allows the sensor to extend outside the relatively large aiming circle, and to still be acceptable. You can overcome this by mapping where your sensor is (which you must do with the Leica as well to a lesser degree), but ultimately, while the Sigs match or exceed it on reflective or very large targets, I found the Leica to be the easiest when it comes to hitting small targets due to the aforementioned precision.
Of course, from a glass perspective, the 2800 definitely has the leg up as well. And it has the added bonus vs the BDX of having full environmentals on board for standalone use, as well as an onboard compass for passing DOF directly to a Kestrel.
But that's not all there is to RF's these days. The Sigs are no slouch in the ranging department, I just find the Leica to be easier to hit the small targets due to the items I mentioned. But when it comes to the other tech, then Sigs take the lead in my opinion. The reasons are as follows:
2400 BDX: Though the onboard limits you to 800 yards (Leica limits at 1k), the integration with the Kestrel (compass not withstanding) is much better in my experience. The BDX connects from sleep much faster (2 seconds vs an inconsistent 16 seconds or so for the Leica), and is much more stable. That especially matters if you plan on having them connected for hunting. 16 seconds is entirely too long to have to wait to range an animal that may only give you 10 or 15 seconds from sighting to getting the shot. And while I don't shoot competitions, in any timed match, I can see this might pose a bit of an issue.
In addition, the BDX integration gives you the option of using a Foretrex, and also connecting with the scope. While I have not used the Foretrex, I have used the scope (I have a review on it if you search). And IMO, that integration is pretty great. Oh, and it's much cheaper than the 2800 or (usually) the ABS as well.
2400 ABS: While it does not connect with the Kestrel, this is still the most integrated of all the devices. No Kestrel with AB necessary...just handle the wind how you want to, press and go. No bluetooth connection needed etc. It does have some things that miss the mark for some (no way to overcome heat drift, clunky way to enter precise wind, though this is a lesser concern), and it still gets beat by Leica's smaller and better aligned sensor in my experience (I don't own an ABS, but have used it, and used the BDX extensively, which has the same ranging hardware). But from a workflow perspective, this is the most fully functioned in many ways, and it is still super capable from a ranging perspective.
The Sigs, they are the more advanced when it comes to ballistics/device integration/connectivity. The BDX connects faster and more securely to the Kestrel, and the ABS doesn't even need that connection (with the exception of the heat drift and wind). It is the fastest and most simple.
You bring up the point of splitting the devices for the sake of upgradability. This is something to consider. RF tech keeps advancing...so with the ABS, you are forced into replacing it all at once. The BDX and 2800, you can replace only the RF or only the Kestrel etc. So yeah, there is some flexibility there.
Basically, I feel like each device has left something on the table, they are sooo close, but still not complete. My advice to Leica would be to work on their programming and get it faster and more stable. Way faster...it needs to connect quickly and assuredly every time.
For both Sigs, I'd like to see them use the smaller divergence sensor used in the 3k BDX (which is slightly smaller than Leica's), shrink down that reticle and improve that sensor alignment! Some better coatings on the glass would be great as well.
For the just the BDX, beyond the above mentioned...compass please. The ABS has it, can't be that hard to add. I also wish it had onboard environmentals like the 2800 and ABS, but if you are paired to a Kestrel, it's not so important, but would be nice to have. However, that would push it into another price class, I am sure, and it' relatively cheap price is a strongpoint you would then lose.
And for the ABS, well, it's got a lot going for it if those things I mentioned as applying to both Sigs were addressed. Add a way to clear the temp sensor the way the Kestrel does, and maybe a more user friendly way of inputting wind (though can be done with a mobile connection, where exact or blanket wind values can be set). Other than that, from a workflow perspective it's the strongest stand alone from these three, but you give up modular upgradeability, and heat sensor seperation.
These are all really good devices. They all range super far. They all provide, one way or another, fast access to advanced ballistic solutions. You just need to think about what you will be doing with it and then prioritize what's important in terms of strengths, and what's not in terms of weaknesses.
Again, not sure I am helping you here, but please don't hesitate to ping me if you have additional questions. I am in the middle of a move, but I'll get back to you as soon as possible.
