I don't know for certain with helm vs MoM. You can't farm stacks quickly with Ursa so that part of the Helm is useless, but Satanic is better than Madness late game. But like I said MoM actually increases your farm rate by a really large amount. So you may end up getting other core items up quicker if you get it. The extra attack speed when overpower isn't up is also really good for killing, obviously.
Also Kirby, what did you go into rosh with? I had stout + Morbid mask and level 3 with 2 in fury swipes and one in overpower. If I ever got low I just waited for Overpower to cooldown and used it to get burst heal. By the time rosh gets you to dangerous levels the amount of heal you get from that should be able to keep you comfortably safe.
And I guess I'm just having Purge rub off on me after watching his vids a lot when I first started playing.
Edit: Having now done it again a few more times, this time against bots set to hard difficulty (so that I would be contested in lane to an extent), it really relies on having an easy lane. You need to get to rosh as quick as you can, and even then, if he bashes a lot early on you are screwed. If you can get enough stacks up early then bashes become less of a problem cus you heal so much from each hit. That said, treants or some other summon would make this ez cus they could tank whilst you get those stacks up.