The answer to your question is "it depends".

If you stay with Marriott, you will be able to accumulate more points in one program. And depending on your number of nights, Marriott does have a quasi-higher elite level, Plat Premier. Not that you will get a whole lot out of that level, but it could mean something for those "big" vacations you mention. Also, Marriott has a lifetime status, so more nights and points gets you closer to that achievement. One of Marriott's best awards is their TP awards (hotel + airline miles). Another is their RC awards. Staying more at the IAD Marriott may allow you to order one or more of those awards. So if any of these thoughts are appealing then maybe staying at the Marriott is better.

The flip side though is maybe right now you are traveling to the IAD area where there is a Marriott. But what if you travel somewhere where there isn't a good Marriott alternative? What if you want to use your awards at a location where Marriott doesn't have any options (or goods ones at least?) What if all the Marriotts are full where and when you want to travel? Then what? Having status and points in a secondary program can give you more options than just one program. But to make this work, you need to be able to stay enough to get and maintain a decent elite level with another program. Also, you need to make sure you can earn enough points in both programs to order the awards you desire. Otherwise, you just dilute your point base and don't get those "special" awards.