An argument in favor of Undumgi:
First, I need to clarify what I see Mathilde as and what aspects of her I want to reinforce: I want Mathilde to be a quintessential grey wizard, with knowledge about events both far and near, able to tip the balance of a critical event all over the world before others even realize there is something that needs to be done, and knowledgeable in matters of arcane in the extreme (basically, Gandalf). In the same time, I really like Belegar and am enthusiastic about k8p being our new home.
Bearing that in mind, let's look at the options: the leading two, which have a very good chance of winning are getting a tower and travelling to Karaz-a-Karak and becoming Belegar's foreign intelligence service. These are conducive for my proposed vision of Mathilde, as they
lock-in k8p as her home
give her an official position in Belegar's court
give her a reason to have an occasional adventure outside of k8p, as stomping thaggaroki and grobi all day every day is bound to get old rather fast.
give her official reason and funding to start building world-wide information network, which in turn would also give access to reasons for adventures in new and interesting locales, without actually leaving k8p
Sure, she might actually do better and be more needed by Belegar as a scout/infiltrator/saboteur/assassin in k8p proper (because she is frankly amazing in that role, and he damn well knows it), but the external spymastering gives us, the players, to a greater variety of things happening, so I am okay with that.
Now, the critical question: undumgi or papers.
Undumgi:
Undumgi have extreme synergy with spymastering externally. Getting establishing them means that we get to establish security over the foreign quarter of k8p (which we need to do as a spymaster), we get immediate access about all the information passing through what is to be a major trade route and have a shoo-in in establishing information network in the nearby Ulrican settlement (again, a necessity).
Undumgi can be leveraged in an increased presence of East Empire Trading Company, as there is world of difference between one of your founders being a dwarf friend living in the hold and one of your founders being in control (for some degree) of the foreign quarter of said hold. This can not only increase EETC revenue and profits but also give us easier time in making EETC a part of our information network, which both helps our general spymastering and lets us keep an eye on Stirland just in case.
Undumgi are likely to be much less sucessful without Mathilde. While they are steadfast and skilled in arms, they are not most organised or law-abiding bunch and are unlikely to have good institutional experience in building such organisations. Anything they come up on their own is likely to be corruption-prone and extortion happy, undermining the very idea of the organisation. Which is a pity, because they can be so much more.
Undumgi do not lock us into being a full-time mayor (unless we want to), which BoneyM graciously clarified.
Undumgi are time-critical. They are being organised right now one way or another, with us or without.
Papers:
Papers are not particularly time-critical. The journemanlings (apart from amberlings) are not going anywhere, they will be staying in k8p for quite some time for reasons of their own.
Papers are a research action. We already have one, and double-dipping in research, in my opinion, is a bit excessive.
All in all, while both options are worthy, the undumgi option seems to be much superior for this round of voting.
Edit: changed uzumgi into proper undumgi. Not sure why I made that mistake. Perhaps the deathwatch meme is too strong?