I'd probably try to find your old Marriott account number, and merge the 2 accounts, and then you'll probably get a new marriott account number - your SPG points should automatically convert to 3:1 (ie, 27,000 Marriott points) and then you'd book your stay with these converted points, although that's just 'semantics' in terms of value (if complex, just call Marriott to sort)
Re Platinum - what you do is login to your AMEX plat online account, find the Card Benefits section, and then there's an area where you can sign-up / link the platinum to gain status in hotels (and hire cars) - you can do this for Marriott (Gold Elite), Hilton (Gold), Radisson, Shangri-La etc, and in case you're not aware, don't forget to sign-up for Accor Plus which gives you a complimentary night's stay every year, as well as Priority Pass lounge access for you and 1 supp card holder - you can also do the same for online Fin Review subscription (if you look for the relevant threads, there's good info on this card's benefits)
I'll leave it to others to add commentary on Marriott vs Hilton - I don't see a lot of value in either program anymore from a 'points value' perspective - AMEX has slashed the value of points transfer to Marriott, whereas it used to be good value, and I'm just not that familiar with Hilton rewards, other than any time I've looked at 'points value' for staying at a Hilton, it appears quite poor.