Peter;
I'm close to finishing a 3-1/2" gauge class 2, designed by Martin Evans, and had the Netta plans drawn by LBSC. I think the 5" one is by Don Young?
Random thoughts – take what you wish from them:
1) LBSC drew about 4 gauges in 1, so chances are that there are issues; (2-1/2, 3-1/2, 5, and maybe O gauge?) so maybe less frustration with the newer design;
2) Boilers – I'd expect the newer 5" 2MT to be better in the design front, but no matter what, you might want to ask your club boiler inspectors;
3) Anything between the frames is a problem to get to for repair, so the less between the frames, the better, in my opinion.
—
Point 3 came from my large 2-8-2 in 3-1/2" gauge; getting at the axle pump is a problem, and it needs looking at again. Something is dripping water under there, and as long as it puts enough water into the boiler, the drips can continue. The locomotive (not including tender) is 120 lbs, so is not a "lift up with one hand and peek underneath" simple inspection.
The 3-1/2" 2MT is designed for 2 injectors, and my 3-1/2" Shay is being completed without anything connected to the trucks (the design has a water pump)
JohnS