Hi, welcome to the forum, they're both good bikes, here's my take on it:
I have both and although there's a lot of overlap I never have any indecision about which to choose for any given ride.
The Mercury is more fun, all that subjective stuff, engaging, rewarding... I Audax and light tour on it, plus use it for all those on-road rides with light luggage where the main intention is to enjoy the cycling.
The Nomad is more sure footed, a bit dull, maybe even boring, though sometimes that's exactly what I want. I use it as a day to day bike, for anything involving off road, and touring that requires more than a change of clothes. It occasionally tows a trailer and if I need to park it somewhere it might get knocked I have the impression it'll withstand that better than the Merc.
If I were to do the same ride on both, I'd enjoy the cycling more on the Mercury and the scenery more on the Nomad, it isn't a question of better but different. Having said that, there's no ride I've done that I couldn't have done on the other, I've done a 200km Audax on the Nomad and a couple of camping trips on the Mercury, I never felt that the bikes were not capable of it, just that they weren't ideal.
Don't worry about the weight, at least nowhere near the top of the list, it's a consequence of design and it's that which effects the feel. rather than the grams. In a similar spec they could easily be brought to within a kg of each other, so in terms of effort/propulsion the difference is negligible. Unloaded, the Mercury is more comfortable, the Nomad is noticeably stiffer, you can mitigate that with tyre and wheel size, but then you widen the difference in other ways. I can swap wheels and forks between mine, get the weight down to a 800g difference, but in their usual configuration there's 3.2kg between them.
The Mercury is my favorite bike, the Nomad my most ridden. Which would suit you better depends on which part of your riding you prioritise, what touring you plan, how much Audaxing, how you ride, what you like... On paper the Nomad is a better do it all bike, but for me a lot of that stuff could just as well be done on a cheap hack, so I would choose the Mercury.
It is mostly opinion of course and there's a nice broad range of that on this forum. We won't all reach the same conclusions, we'll all have our own preferences and subjective tests, though the physics is the same for everyone.