I can offer limited advice, but since there are no responses to your post as of yet...

#1. Six weeks will give you 40 something nights, which is a tad short of becoming a gold member. I've seen elsewhere here where there are offers for an expedited gold membership for which you should qualify if you sign up for the offer.

#2. Residence Inn properties, while nice for long term stays only give you half the points of other Marriott properties (5 points per dollar vs 10 points per dollar). So if points is a priority, note that you'll get half as many if you stay at a Residence Inn.

#3. Check for any megabonus offers, because staying at a Marriott for six weeks straight you should be able to qualify for some of those.

Good luck