My extended family has lots of birthdays in September, and the family was planning on getting together to celebrate two birthdays that were on the same day, and when they realized that my sons' birthdays (as well as another person) were also within a couple of days, we decided to have a joint family celebration for all 5 birthday boys... That clinched it- if there was going to be a joint family party, I wasn't also going to make two birthday parties for my boys in addition- I'd do joint for them as well (and they were cool with it).
So, I asked them what theme they wanted for the birthday party, and they both agreed to a race car themed party.
I've written previously about why I like doing themed parties, but in short, I find we can have more fun and make the party cheaper as long as it fits a theme.