In the ES7 spec, which RxJS 5.0 follows (but RxJS 4.0 does not), the two are NOT the same.
subscribe
public subscribe(observerOrNext: Observer | Function, error: Function, complete: Function): Subscription
Observable.subscribe is where you will do most of your true Observable handling. It returns a subscription token, which you can use to cancel your subscription. This is important when you do not know the duration of the events/sequence you have subscribed to, or if you may need to stop listening before a known duration.
forEach
public forEach(next: Function, PromiseCtor?: PromiseConstructor): Promise
Observable.forEach returns a promise that will either resolve or reject when the Observable completes or errors. It is intended to clarify situations where you are processing an observable sequence of bounded/finite duration in a more ‘synchronous’ manner, such as collating all the incoming values and then presenting once, by handling the promise.
Effectively, you can act on each value, as well as error and completion events either way. So the most significant functional difference is the inability to cancel a promise.