Let's say that you have a paper that you've recently presented at a conference. Your plan is to combine this paper with another conference paper, with some additional extension work thrown in, and submit it as a journal article. (This is pretty much standard practice in CS.) And in fact, you've already started working on it. You feel that the work is strong enough that it has a good chance of being published in a pretty good journal, maybe even one of the "big" journals in the field.
Let's say that you were contacted by the organizer of that conference, inviting you to submit an extended version of the conference paper to a different journal. The paper would still undergo the peer review process, so no guarantees that it would be accepted; but you assume that the odds are pretty good that it would be accepted. You are not sure that this other journal is the best fit for this work, however; in fact, the extensions in order to make it fit to the journal may be tricky (or at least not something that you pictured doing with this particular line of research). And then there is the whole dilemma of what do you do with that other paper, the one you were going to combine with this one. (Most likely, the paper would sit around until the next conference paper gets written, which may not be for a while....)
If you were in this situation, what would you do?