It's really hard to decide... comparing those bags is like comparing apples and oranges.
I honestly didn't know the black coffer is hard to find. I thought this was the staple color and the coffer is a classic design so I was under the impression it would be around a while. I could be completely wrong of course, but I'm sure someone here at the forum can shed more light into this.
Don't get me wrong, the coffer is TDF. Linpaddy described it wonderfully and it is one of my most favorite purchase! I also plan on getting more of the coffer in other colors once I can afford to. I would definitely recommend purchasing the coffer to anyone!
The pink tote you showed, which Linpaddy posted earlier, is also beautiful. I believe this is a seasonal style so it would definitely be harder to get one by the end of the season, I would assume.
If it were me and I had to choose(this is under the assumption that miumiu will keep producing black coffers, of course), I would pick the seasonal bag... only because I've had sad experiences when it comes to passing over bags I could no longer get. I could always save up for the coffer as my second purchase (it never stops at one).
However, if it turns out that the black coffer is in any danger of being phased out, I would get it in a heartbeat.. no questions asked.
I just reread your post... and really, if you've been lusting over the coffer for more than one year... get it! Don't let the new fads/styles sidetrack you - otherwise, you'll never be able to get it. You won't regret the purchase either way. The coffer is a really special bag... It's one of the reasons Miu Miu is still around.
Good luck with your decision. Don't forget to post pics of whichever you get.