I think as a resale purchase, II can be a very important tool. The reason I say that is because you don't have the option of converting your unit to Star points to use for hotel and/or airline use. So, if you are unable to rent your week or use your week your only option is II or to let your week go. So, if I had only those two options then, II would be my choice. Things will happen in life where you won't have the option but to deposit the unit in II, etc..
In II, you have from one year prior to the check-in date to 2 years after the check-in to exchange your unit. I would think with a Westin Maui unit you should have no problem exchanging a studio for a one bedroom & a one bedroom for a 2 bedroom. I would definitely trade as separate units so that you get 2 trades for depositing a 2 bedroom unit.
I have never traded my Maui unit yet but I do own Marriott units that I have traded so, I understand a lot about trading. Planning is the key. Plus, researching is the key as well. You can easily figure out if your trade is possible just by understanding the system and how it works. You also, need to understand how the other system that you are trading into like Worldmark, Hilton, Hyatt, Marriott, etc.. work so, you can understand if a trade is possible. Alot of people complain about systems but the truth is they don't understand them. So, instead of trying to follow the rules they want to change how the system works which gets them nowhere.
While, many here don't like II it is still a valuable tool when you have no other options. I would never let a unit go unused for a week if I had other options even if the value isn't there. I think that would be silly but that's just my opinion.
Good luck on your search!