Is "it" used correctly here? Does it clearly refer to the job?
Landing a better job is one thing. Retaining it is a different proposition altogether.
I think that it would be better if we repeat "the job" because it might seem that "it" refers back to "one thing" instead of "the job"!
Landing a better job is one thing. Retaining the job is a different proposition altogether.
The word "it" is used correctly there, but you're also correct that it's often a good idea to specify what "it" is, which unfortunately has the drawback that it sounds repetitive to say the word "job" twice so close together.
Both of your proposed sentences are grammatically correct, and perfectly acceptable. Which one to choose is just a matter of preference. If you need the sentence to be shorter, you can use the first one, and if you worry that the person with whom you're communicating may have an intellectual disability or something that would make it especially important to be less ambiguous, then the second sentence might be better. The extra detail in the second sentence might also be helpful in some contexts such as legislation (law) and things like company policies, where ambiguity ought to be avoided as much as possible.
Landing a better job is one thing. Retaining it is a different proposition altogether.
Both versions you have suggested are grammatical.
As the context is clear, I think chances of it being interpreted as refering to one thing could be low. Besides, Retaining one thing is a different proposition altogether would sound weird here.
Hence, the original version could be better as it avoids repeating job.
As the two clauses are closely related, we could consider placing a semicolon instead of a full stop between them.
Landing a better job is one thing; retaining it is a different proposition altogether.
