I have done both: the LQS ones where you buy in and make the current month, get the next month free as well as the online ones.
Sometimes the onlines are also the only place to get the pattern. When I was working, it was also easier to have them collect the fabrics and cut them the amount needed, especially when it comes to very small pieces (less than a fat quarter) than buy more than I need. Sometimes it also seems less painful for expensive patterns (like the ones with a lot of applique or paper pieced) if you do BOM and are getting the fabric, especially fabrics you may like, but are not necessarily what you would buy on a regular basis.
It all depends on what you want. Since there are now more websites with free ones and more stores offering them, there is more to choose from. I am currently doing a make one, get one BOM at the LQS (designed by them), but I am using different fabric than they provide.