Agree, it really depends on what one's travel needs and goals are. If you have use for 70K Marriott points, and don't have a particular near-to-mid-term need for 30K AA miles, I'd leave them sit. On the other hand if you could make use out of the miles more than the hotel points, make the transfer. A good deal isn't a good deal, if the end result doesn't provide you with a useful benefit.

Me personally, I have no real use for Marriott or most hotel points based on my travel patterns and plans for the next year or so. All other things being equal, I'd get much more out of the AA miles. However I still wouldn't do this transfer as I have enough AA miles for now and am planning to use other miles in the nearer term. We now have the ability to transfer Marriott to SPG points (1 SPG per 3 Marriott), and SPG has (for now) many airline partners (including AA) and the 5K bonus per 20K transferred. So I'd probably leave my points alone for when I need some flexibility to top up balances in other airline programs when the need arises.

Just my perspective - there's no right or wrong, really - just spend some time figuring out what's right for you.