Remember that "establishing" a QOF doesn't actually do anything. The regulations make clear that having formed your QOF, you can then delay literally declaring it a QOF and making your investment. So, if you know that you are going to form a QOF, then setting up your QOF right away (but not capitalizing it) saves the risk that your legal/tax professional goes on vacation (or gets busy) when you actually want them to write you an agreement and have it qualified under local law.
However, "establishing" is different from "investing." So, if you don't really know what you are going to do with the money, then presuming that you take care to get your investment in by the 180th day (after the capital gain for portfolio gains and 12/31 for Section 1231 gains), delaying the actual investment gives you two benefits -- (1) the 6-month period for testing the QOF's compliance with the 90% test is delayed, and (2) you can use the money to pay for something else (e.g., buy a boat, and then borrow the money for your QOF investment at a later date) or simply invest it in something that is more profitable in the short term (e.g., the stock market). Once the money is actually in the QOF's name, there are pretty strict limitations on what you can invest in.
Of course, there are look-back rules for computing the 90% test at the QOF level (you get to ignore amounts received in the past 6 months if they are invested in cash-like investments and short term debt), and similarly, the 70% test at the subsidiary entity level is passed if you put the money in such cash-like investments, provided you have a 31-month plan to deploy the capital on a qualifying project.