I have a down coat that I think is meant to be knee length but is actually midi length on me (hits midcalf) because I am short. This has been my only winter coat for a few years, and on days that are really cold (like below zero F), I'm so grateful for it! The extra length provides much-needed insulation. The same goes for days that are extremely windy--the extra length is a great windbreaker and keeps me so much warmer. Everone else is freezing, and I'm like, Huh? I'm fine.
However, having said that, on milder days, say, in the 20s or 30s F, the bulk of this coat is a killer. I get exhausted hulking it around and walking when I'm on errands. For that reason, I am looking for another puffer to complement the long one that I have.
I would personally recommend looking at what your weather tends to be, and if it is often very windy or extremely cold, have your main coat be the longer length, and get maybe a cheaper shorter one, and if your weather is more mild, I would spend your dough on a nice shorter one and buy a cheaper longer one. I really think that if you have a winter of any reasonable duration, you need two lengths.