The best answer is to have your buyers agent research past sales and then you will see how much the developer took off list for similar units (or if they all sold for full price you probably know to go for credits instead). I hope this helps. Good luck!
Your best bet is to identify the unit you like and then have an agent help you negotiate. An experienced agent with a lot of background in representing buyers and sellers of new developments is a skilled professional worth using. You have nothing to loose by submitting a contract.
Best of luck
It's difficult to say without knowing more about the particular development and the specific unit itself. Sometimes there's no rhyme or reason why certain units sell before others. I've sold units in projects that we thought would take longer to move but ended up selling first and units we thought would go quickly ended up taking longer just because the right buyer hadn't appeared until then.
If a unit is one of the very last available you need to take into consideration why that's the case. If previous buyers bypassed the unit in favor of others, you may be getting into something that could be difficult to resell at a later date (views, size, floorplan). Alternatively, it may be a great unit that was initially overpriced and is now at current market levels. The developer may be more negotiable depending on current circumstances especially if they're looking to get involved in another project and move on quickly from the last remaining units.
Ultimately, you never know what the answer will be until you put an offer in writing for a specific unit and receive a counter-offer. If you don't like what the developer will sell it to you for or the credits provided, move on.
Good luck, Ken.