I belonged to an FSSP parish for several years, while periodically also attending the SSPX. Both are quite far from where I live, and logistics and transportation concerns had a lot to do with it.
After returning full-time to the SSPX several months ago, because their position is the one I with which I most concur, imagine my surprise to find how much the Society had changed and how similar it was to the FSSP that I had left. That is why I now attend the Resistance Masses wherever possible.
As with the SSPX chapels, there is quite a bit of variety depending on the location in the FSSP. So I wouldn't make any sweeping generalizations. However, a good reference point for both groups is their website.
Lately there have been some really awful sermons at the nearest SSPX chapel, and I can honestly say that I have heard many that are more hard-hitting and inspirational at the FSSP, though I have no intention of returning.
Yes, there are some excellent priests in the FSSP, and I was blessed to have one of them as a confessor, but even he was operating within the framework of an organization that was under the local bishop. That has some far-reaching implications that I suspect many SSPX priests and faithful have not considered.