There is still a big place in my heart for my L300 but I have to say I LOVE my L400. Yes, the 400 SWB is smaller inside (behind the seats) by about a foot because the seating is much further back than the 300 but that is not a big deal for me. More important offroad is the approach angle and departure angle of the SWB versus the long wheelbase. Departure angle is sacrificed a bit on the long version but a lift can help that. Approach angle for L400s is much improved for the 400 over the 300. Another important improvement is the L400 is very 'clean' underneath... the L300 transfer case hangs low and can be bottomed out. The L400 has everything tucked up nice and clean... if you bottom out, you bottom everything out but you would pretty much be sitting on the frame at that point.
The greatest difference though is the power on the highway. The L300 is sluggish on big hills and you have to take care not to heat up the engine too much - let the van tell you how fast you can go - could frustrate the wife on road trips - trust me on that

I have no qualms about taking my bus offroad into the bush. Can things break? Sure! But... they can break on any vehicle and you would be equally stuck out there - no difference. No parts stores in the bush. Always carry tools and recovery gear and travel with a buddy vehicle in case you get in a pinch.
My wife really loves my L400 - and she isn't an easy person to sway. It has enough 'luxury' to satisfy the feminine need and enough 'utility' to satisfy the masculine need. Good luck in your search!
Cheers!
Ian