If I understand your question:
If you purchase a 2BR lockout unit, you can either use (or trade) the entire 2BR unit all at once or split it (lock off) the week into a 1BR Master unit and a separate efficiency unit. (There is a small fee to lock off the unit.) If the unit is split, you could for example stay in the 1BR or the efficiency but trade the other portion of your unit for a second week of vacation (expect a trading fee).
If you purchase a dedicated 2BR unit that cannot be locked off, you will have one week of timeshare usage with that 2BR to either stay there or trade the entire 2BR.
The "trading power" (the ability to make a desirable trade or a trade into another 2BR unit elsewhere) will generally be greater if you deposit a 2BR unit compared to a 1BR Master unit. And a 1BR will have more trading power than an efficiency. However, many Marriott units trade pretty well. My Marriott week is a "stronger trader" than my Sedona Summit week. So my Marriott efficiency will pull better trades than my Sedona efficiency.
I own a 2BR lockout (I am not a points owner) and almost always lock-off. I like the flexibility and prefer it that way, but it is a personal choice. From the way you phrased your question, it seems that you would as well. Still, if you are buying a resale week (rather than a points purchase direct from Marriott), you want to buy into a resort you will enjoy.
Another thing to understand: let's say I desire a 2 week (back to back) stay at my home resort. With Marriott, I cannot reserve week 1 in the 1BR and week 2 in the efficiency. I have to reserve my entire 2BR unit all at once, lock it off, keep the 1BR portion for my use and then try to trade the other portion (let's say the efficiency) and pick up the following (or prior) week (hopefully as a 1BR Master - it's nice to "trade up"). (But I can reserve back to back weeks with my Sedona Summit timeshare if I wish. Different resort companies have different rules.)