2000 31' or newer 29'
Couple thoughts come to mind based on what I have heard and seen about the two units --
1. The newer the better. Older units will be more likely to have leaking issues, more rusty frames, tank sensors malfunctioning, upper/lower gaskets needing replaced, possibly replacing cables, Fridge issues, water heater issues, etc.
2. The 31' had a 'slide out' as opposed to a tipout in the 29'. The Slideout is a much heavier assembly. I have no knowledge of its reliability. The Tipouts are generally reliable.
3. I have heard over the years that the 'really long' Hi-Lo's often suffer some 'frame bending' -- particularly closer to the front. The 31' could have this issue. There is a fix for it, but it involves cutting a piece of the frame out on both sides to straighten it, then plating over the cutout. I have not heard of the 29' having this problem. There apparently was even a 33' Hi-Lo, but is extremely rare, and would be very susceptible to this problem. The older trailers (maybe before 2005 or 2006) have boxed frames. HI-LO since then went to I-Beam frames, which are stronger and lighter. The bigger the trailer the more important a strong frame is. For instance -- my 1508T uses the same dimension frame as the large 28' HI-LO. It is over engineered for the 15', which is a good thing. The only thing that is different is the tongue, which is much sturdier on the larger HI-LO's.
Hope that helps,