Arjuna wants to know which one is closer toward moksa between a karma-yogī who meditates upon cosmic form of Īśvara or a renunciate who contemplates on attribute-less-brahman as oneself. From the question itself shown that the meaning of renunciate is still not clear for Arjuna. He might think giving up karma as akarta (non-doer) / abhokta (non-enjoyer) is by will, that means renunciate is by will. One can take formal sannyasī just like what is stated in Gītā chapter 3.3, "there are twofold of commitment of lifestyle for pursuing moksa, one is living a life of karma-yogī who keeps one's duty in the same time pursuing self-knowledge, and a lifestyle of sannyasī who renunciate his/her duty exclusively pursue self-knowledge. Even though choice is given here, but for one to take formal sannyasī, one must have done enough karma to gain sufficient amount of maturity, then sannyasī's role can be proper and fruitful. This fact is supported by Gītā chapter 3.4, "it is not action-less by not performing action, and one is not liberated by renunciation." And the culmination of both lifestyles is jñāna-karma-sannyasa as stated in Gītā chapter 4.18, "one who sees action-less in action and action in action-less is wise person." Renunciation happened because of abidance in self-knowledge, owned up the truth that I am limitless Brahman. There is no more doer-ship and enjoyer-ship for him/her, every action is done because of the order of Īśvara. This sannyasī is the one who can see nirguna-brahman as oneself, which Arjuna tried to compare with karma-yogī in his question. There is no scope of choice in this case. Next are the laksana - implied meaning of Brahman where one can see these meaning is oneself alone.
aksaram - which is not mutable, never subject to change. avyaktam - not available for objectification as an object of perception, because not manifest as an object of any pramāna (means of knowledge) - aprameya. Since it is not an object of perception, we can't point it out as this or that, not describable - anirdeśyam. If no words can describe Brahman, what is this study about? Even so, we still use the implied meaning to arrive at the understanding. In Vivekacūdamani says that "sarva-vedānta-siddhānta-gocaram tam agocaram" - that which is not available for any kind of objectification yet revealed by all the Upanisad; it is their siddhānta (final establishment) which is tat tvam asi - you are that Brahman.
In fact, there are a lot of thing in this world also can't be described, for example sweetness. The only way to know sweetness for someone who never taste it, is to taste something sweet. It is not describable but still, it is perceptual experiential knowledge. Is it the same case with Brahman? It is not so, because Brahman is not any concept of the mind - acintyam, since any experience is just a concept of mind. Brahman is not the object of experience nor the experiencer (mind), but it is the very svarūpa - nature of the experiencer itself, which is always present no matter experience is there or not. Therefore, how is Brahman going to be experienced by ātmā - me? It is the very svarūpa of ātmā - me.