I much prefer IPS and I am very much loving all of the new features coming with IPS 5. Granted, the underlying software doesn't matter too much for forums anymore and XF is much more popular among small forum users. Also don't forget that IPS 5 is also removing the ability to buy pieces of their package, so you will need to pay for all the apps (granted the overall price is going down, but it is still more than just the forum app).
I do believe that IPS 5 will beat XF in just about every way, but as a long time IPS admin, I can honestly say that users won't easily switch software preferences based on better features.
Personally, I'd take note as to where you are marketing and getting users from, if it is all XF, then factoring in getting them to use IPS is a hurdle. However, the software won't make it break the future of the site, so the choice is really yours.
Granted, if you want to use other IPS apps, XF has little to offer in return and nearly all their answers are 3rd party add-ons or simply don't exist. This is the primary reason my sites run on IPS as they simply can't exist on XF.
