You will now note the distinction between the Watchful and the Clean man (although they are closely related). The first is Watchful of his deeds and sees to it that he does not sin in relation to what he knows, and what is universally acknowledged to be sinful; however, he is still not so much master of himself as to keep his heart from being pulled along by natural lust and inclining him to rationalize in relation to things whose evil is not thus acknowledged. For even though he exerts himself to conquer his evil inclination and to subdue his desires, he will not, because of this, change his nature; he will not remove bodily lust from his heart. All he will be able to do is overcome it and be governed, not by it, but by reason. The darkness of earthiness, however, will still persist in its work of persuasion and deception. But when a person habituates himself to Watchfulness to the point where he completely cleanses himself of the acknowledged sins, and accustoms himself to zealous Divine service so that love and yearning for his Creator grow strong within him, then the force of this habituation will draw him farther from the realm of earthiness and direct his mind towards spiritual perfection. Eventually he will attain to perfect Cleanliness, a state in which physical desire is extinguished from his heart through the strengthening within him of the longing for God. His vision will then possess the purity and clarity that I spoke of above. He will not be deceived, he will not be reached by the darkness of earthiness, and his deeds will be absolutely Clean.

...Likewise with regard to all the pleasures of this world: the wise man foresees what becomes of them; they ultimately rot and become worms and refuse.Conversely, one should train himself to delight and rejoice in G‑d, by reflecting, to the best of his ability, on the greatness of the blessed Ein Sof.He may well know that he will not attain this degree of loathing evil and delighting in G‑dliness with the fullest measure of truth, but will only imagine it. Nevertheless, he should do his part to uphold the oath administered to him to “Be a tzaddik,” And G‑d will do as He sees fit — whether to grant him the level of tzaddik, or not. Furthermore Habit reigns supreme in all matters; it becomes second nature.Therefore, when one accustoms himself to loathe evil, he will begin to find it truly loathsome, to some extent. And when he accustoms himself to rejoice in G‑d through reflecting on His greatness, then, [on the principle that] “an arousal of man below brings a corresponding arousal above,” perhaps after all this effort of his, “a spirit [Ruach] from above will descend upon him,” and it will be granted him that the [soul-level of] Ruach, originating in the soul of some tzaddik, will be “impregnated” in him, so that he may serve G‑d with joy.Thus is it written,10 “Rejoice, O tzaddikim, in G‑d.” In this way, the oath charging him to “be a tzaddik” will be truly fulfilled.

Mesilas yesharim ch. 10

He writes:
How is this different (or simelar from what it explain in the Sefer Hatanya chapter 14 (and later) ?

Which came first?

but I am not sure they are saying the same thing (not that it changes anything if they were).

I'm not sure the section of Tanya you cited is saying the same thing. But what the Ramchal says here is definitely to be found in Chassidic Seforim (including Tanya but I'm not sure all in one place).

There are two issues here 1. The importance habituating oneself in Torah and Mitzvos until you get to the level where serving G-d is almost second nature.
and 2. The importance of ridding (or fighting against) taivos olam hazeh as much as possible in order to advance in the service of G-d. And the inability to properly serve G-d or reach a high level of understanding anything spiritual if you are drawn to the physical...

Wouldn't habitual service lack the vibrancy and life a truly committed individual's service has?