In the case of an iPhone you won’t be able to detect a difference in charging performance in a QC3 port because there is a negotiated common denominator of 5V 2A 10W and the iPhone will struggle in most cases to use as much as 2A.
It is a little different in larger devices like iPads, the smaller tablets can take about 2.3A so they’d be a little 10% slower charging in a QC port rather than certain Anker chargers which output 2.4A. Larger iPads can draw more and then you really need to go to USB-C.
So as you only mentioned iPhone then you won’t really be worse off on performance, but you would be worse off in your wallet and size. QC chargers are on average more expensive, larger, heavier so if you did not own a QC product (usually an Android phone such as from Samsung) then you’d usually find you’d be fine with a non-QC product.
QC in the long term has elements of being doomed as its only really a phone charging technology, and the future is looking increasingly pro USB-C. Apple who are key player in phone and tablets is moving towards USB-C across its range. USB-C lowest common denominator is like QC of 2V 5A 10W. So over time as devices need more power the usual situation a QC is no worse for an Apple product will become QC is definitely slower than USB-C PD…
If you don’t own anything QC now and seeking to make an investment then consider USB-C PD. The one I own is the Powerport5 USB-C which has a single 5V 3A 15W port which makes my larger tablet’s needs. Anker’s USB-C PD products are somewhat lacking at the moment and we’re waiting for the “soon” promises to manifest. So there isn’t that much to invest in USB-PD from Anker at this moment.
Also as the future isn’t written yet you are ultimately best making a decision on what is available now and in general the lowest cost approach on what you have now is more likely best, so if you have only iPhone then practically all of Anker’s chargers are fine, and probably its a choice between the Powerport2,4 (no cord, plug into wall directly and you tend to pair with longer USB cables) and Powerport5,6,10 (which have a cord which goes to the wall and you tend to pair with shorter USB cables).