Let's look at all the angles here. The first is what is best for Madoka. With Homura potentially waiting at the park, she'd be uncomfortable to take down Kyubey, thereby heightening the chance that we make a wish considerably, which is of course deeply problematic and the thing we want to avoid the most.
On the other hand, going would mean that Sayaka is incredibly vulnerable to being forced to contract if the witch goes rampant, Kyubey isn't overly likely to target her- she doesn't have that much potential, but it is still a definite concern.
On the third hand, Metagaming is bad and we really should keep to our promises while Sayaka goes for her boring shop run, so... xP
[X] "Sorry Sayaka but I have some things that I need to do first."