A wrong choice of airline could see someone doomed to be a career F/O. It happens for a variety of reasons, not always due to any lack of ability. Airlines go belly-up, for example, then you start again somewhere where they run a strict seniority system, or maybe they need you to have prior command time to upgrade because of insurance or client requirements etc.
So, my advice - for what it is worth - is to look past the aircraft type and decide on the basis of how you see the risks - a stable and profitable turboprop operator, or maybe a jet operator with equipment that is rapidly becoming obsolete and no replacement plan?
So often now it seems that operators require recency on a given type, rather than generic experience. Are there more ATR-72 jobs than B737-400 jobs? If you did have to convert to, say Airbus, would it be easier to do it from an ATR or from a Boeing? It is unlikely that either path would be any more difficult than the other, if you are competent.