We have used both tents extensively: had a Lunar Duo for six or seven years, and about wore it out. Got a Hexamid Twin for our LT hike for the much smaller pitched footprint, but it's really too small for the both of us. So we got a Triplex a couple of years ago. The two tents are broadly similar: two doors on the long sides, two vestibules, lots of interior room. I'll focus on the differences in use (you can figure out the differences in weight, price, etc., yourself.)
Lunar Duo: The curved poles on top of the doors seem to add extra interior headroom, at least it feels roomier above my head. The vestibule is easier to use with the zipper down the middle. The Duo is slightly harder to pitch consistently well, as very slight differences in angles make a difference. The Duo is exceptionally weather resistant. It does stretch and sag when it gets damp.
Triplex: more complicated to pitch, but once I got the hang of it, I could do it consistently, and pretty quickly. It's wider so there is more room across the middle, but it seems to have less headroom and also less usable length (so, for example, I can't get as much gear and stuff piled above my head.) The Triplex fabric does not absorb water, so it can be shaken off better than the Duo. It does take more room to pitch than the Duo. This tent is also quite weather resistant, though I've not had it out in the kind of intense thunderstorms that I experienced with the Duo.
Both: Both tents seem to pack down to the same basic size. They both get a fair amount of condensation under the right conditions. They are both excellent tents for a hiking couple.
I think in terms of being stupidly sentimental about gear, I *loved* the Lunar Duo. Just a great wonderful amazing tent. We had some great adventures in that tent, lots of good memories.
But my knees love the Triplex.