Perhaps a bit of confusion there. First, there are actually
two model lines with "L" in the model number... the "basic" L's, with numbers ending in "00" (e.g.
L4400) and the higher-end "Grand" L's, with numbers ending in 10, 30 or 40 (e.g.
L3940), with the 40's being the current generation. Please don't ask me to rationalize the numbering beyond that because I can't. However the Grand models are somewhat larger and have more hydraulic capacity, more lift on the 3 pt, heavier front axles on some models, stronger loaders on some models, the new HST+ transmission on hydro models, some comfort/convenience features and a price tag large enough to cover the added content.
The MX4700 and MX5100 have the size and power of a comparably powered Grand L but have more basic, utilitarian features. It is definitely larger and more powerful than an
L4400. The backhoe issue has nothing to do with the MX's not being sturdy enough. That configuration is sold in the states and is a very strong unit. There is reportedly some issue with getting it certified with the Australian safety agency (we call it "OSHA" in the US, although there is usually a four letter adjective or two added to the description). There is another Aussie who has been wrangling with this issue of late. Maybe he'll chime in here or you could search some of these threads to get more information.
The MX is available with HST, but not the latest HST+. My tractors have the older type HST, which has been excellent, but everyone with the newer setup seems to rave about its performance.
Can't guess what motivates your dealer beyond availability of another product or lack of experience with grapples. You can read up for yourself what owners have reported here. Best of luck with your deliberations.