You can decide when you're there if you want to purchase express pass. There are two kinds of EPs you can buy - the more expensive one allows you to ride an unlimited amount of time through the EP queues. The cheaper one allows you to ride once per ride in the EP queue.
As PP mentions, Universal is now using virtual queues for most of the popular rides which is separate from EP. You need to book a time for the VQ, similar to WDW's FP. However, if you have EP, you don't need to worry about VQ and can simply enter the EP queue when you want to. Crowds and wait times have been low recently, but there were some reports during a recent busy weekend shortly after reopening that said many people were having trouble obtaining a VQ time. They were filling up. Without a VQ time, you can't ride that ride because VQ is the same as the standby line. So, in those cases, EP would be beneficial because a VQ is not needed to ride. I have not heard of that happening since that particular weekend, but something to watch.
I would simply wait and decide on the fly while you're there if you want to go to Guest Services to buy them. See how wait times and VQ availability are looking.