I'm sure you bought a tractor by now, but I just wanted to reply for others who see this.
I'm a mechanic for a LS dealer and in my opinion I prefer the new LS engines over the older Shibarua engines. The LS engines regen a LOT less because they were designed from the ground up to be a tier 4 final engine. The Shibarua is a old mechanical tier 3 engine that was converted to tier 4 interim. I find that the LS engines are more reliable (especially later years as they worked the bugs out) and more powerful.
If you live at higher elevations I definitely recommend the newer LS engines because the computer will adjust the timing for the elevation that your at since it is a common rail system. The other engines have to have the injection pump shimmed, and have more regen issues at elevation due to less air from what I under from other dealer techs. We sold a tractor to a guy a XR4046C at about 6000 feet and ended up trading it for a XR4145C because the XR4046C had no power at all at that elevation.
I doubt their are any new older tractors at dealers anymore, but if there is I would still with the newer ones. If you get a smoking deal on a used one I guess it is ok. But go with the new style if you go new.