I know nothing about ATG (except seeing a
recent post that seemed to cast doubt on its future), but I am certain that deciding on a job based on the use of a single framework or technology seems a bad idea. That kind of decision hinges on so many more factors particular to you -about which we know nothing- that folks here can't really give a meaningful opinion on that (except maybe to state that it has no future so
you should not join no matter what).
