As a former SMF user, I can tell you there is good and bad to both systems.
Please keep in mind that the following is based on my own experience and my own opinion. Some of you may or may not agree hence why this is all an opinion.
PROS of SMF over XenForo:
On SMF you can easily use the Reputation (Karma) System as it is prebuilt into the software. You can also name this anything you want and restrict who can and cannot give out Reputation Points thus you can create your own unique point system from this. I turned my Reputation System into a "Participation Points" for events and tournaments I used to run. It made it easy to track users that participated in my events. On XenForo, you will need an Add-on to do this.
With SMF you can also easily use Rank Images based on Post Count (Such as the traditional user rank star.png images) and you could easily change these stars into anything you want. XenForo does not do this.
Permission settings were a lot easier to manage on SMF. Xenforo tries to make it easier by changing one permission to affect all users but in the end, I find this more tedious and it is very easy to miss a simple permission which restricts you from doing something (yes even super admins can be restricted from doing something on XenForo whereas on SMF, super admins have total rights and cannot be locked out of anything.
SMF uses traditional naming conventions such as a forum is a forum. This makes it more user non tech savvy user friendly. XenForo uses the name "Node" as a forum which can be a bit confusing for first timers. It took me a while to figure it out.
PROS of XenForo over SMF:
Modern and more "this" generation type feel. Xenforo will relate more to this generation with it's advanced Like and AJAX features whereas SMF feels more outdated.
XenForo Addons rarely ever interfere with each other. In SMF, you can download a thousand Add-ons and get a thousand errors because the developers don't bother to build their Add-ons to work with other Add-ons. Here on XenForo, developers build to compensate others and when an error is found, it's usually fixed relatively quickly.
XenForo has built in Search Engine Friendly features which is just always better. Period.
You can manipulate almost anything on XenForo from the ACP with some knowledge of coding. If you don't have Knowledge you can ask here and most people are generally willing to help you figure it out.
The Community is overall more friendly and more helpful here than they are on SMF, however, you can generally find someone willing and able to do custom work requests on the SMF board a lot quicker and easier than you can here.
With that said, I would not nor could not ever revert back to SMF. It just feels way too old now when compared to XenForo. I would highly recommend XenForo over all the rest.
PS- Most of the SMF Add-ons are built and only work correctly on the Default theme. So when you have a custom theme as most people do, you have to do some extra work to get those Add-ons to work on your theme. Here on Xenforo, it does happen, but not quite as often and usually when it does, the Developer will try to help you fix the issue.
Good info here. I'm pretty sure we'll be going with XF when the time comes. 
