I was considering CCM as a back-up should I have not gotten into EM. I decided against it after actually rotating in it, which made me realize some major differences with it and EM.
The majority of your patients in the ICU will be on the ventilator. Even those who aren't are so sick that you can't talk with them much. One of the best things about EM for me is that you get to talk to patients, meet new people every day, etc.
I'm a social person and I like talking to patients. During my IM rotations in my Transitional Year, I spent like 30+ minutes going through the patients charts, labs, filling out crap, etc., and only like 5 minutes in the patient's room. In the ICU, it was like 3 minutes with the patient and 45+ minutes doing other stuff, like filling crap out, looking through the charts, labs etc. (I'm talking about pre-rounding on patients.) I feel like in EM you spend a greater percentage of your time with patients, and less time writing crap, going through charts, etc. This I speak from the perspective of an intern. (Maybe EM attendings have to deal with a lot of paperwork, I don't know.)
Of course, you'll get your share of ICU-like patients in the ED, but it's not the ONLY patients you meet. It's a healthy mix of patients, from super acute to why the heck are you here in the ED. I like that mix, instead of having all on-the-brink-of-death patients who can't talk, communicate, etc.
Also, all that buzzing and beeping in the ICU would drive me nuts.
I may have misunderstood your question. If you are asking what route to take to do CCM, I think doing IM would be the best route to take, not EM. Whatever you do, I think you would want to not always have to defend your training. For example, I would never advise a person to go through FP and then do an ER fellowship. You'll always be second rate that way. If you do EM and then CCM, you'll be looked down upon by your colleagues. For some, this wouldn't be an issue, and all the more reason to excel...but for me, I'd hate that situation.
Also, if you do end up doing the CCM route, do a Pulmo/CCM fellowship (it's just one year longer). I know of at least a couple people who did that and ended up focusing on the pulmo part, since they hated doing CCM in their later years of life. So it's good to have that option available. Although I think CCM will eventually shift to hospitalist type of schedule--or even like EM--but right now, most CCM jobs seem pretty hectic and unrewarding...that's what I hear from very trusted people in the field. Lifestyle wise, CCM sucks. On the other hand, both EM and Pulmo are not as unfulfilling in that aspect.
Anyways, for me, EM >>>>> ICU.