Reinforced toplink bracket... offered separately by Kubota for the L10 series. Check with dealer, but I don't think the new L30 tractors need a stronger bracket since the 3pt lift and toplink were redesigned in the new models. (suggest you update you TBN profile... it indicates you have a 3710... congrats. on new 3130, though!).
Maybe some of the backhoe owners (vs. an owner wannabe like me /forums/images/graemlins/crazy.gif) can better comment on whether the 3pt or the subframe hoes are easier to put on and take off. I had the apparent misconception that the 3pt variety would be easier, but my dealer opines that the subframe hoes are actually somewhat easier.
With most of the hoes available of either type, you do have to actually remove the lower arms to install the hoe. So, putting on the hoe is not quite the same as hooking up your boxblade.
I've also learned the obvious point that ease of installation varies between makes and models of backhoes. I think some or all of the Bradco hoes let you leave the lower links on, for example.
A subframe mount is certainly stronger than the 3pt ones, but as you say, it does reduce ground clearance, and stays with the tractor even when the hoe is removed. Again, some manufacturers apparently do a better job with this than others. Just going by what others have said, though, if you plan on heavy use, better go with the subframe type.