I have the first two volumes of "Simple Steps to Sight Reading " from TalkingBass and I am also an SBL Member. I got the TalkingBass courses long before I became a member at SBL. For that reason I have not gone very deep into the sight reading lessons at SBL. But what I can tell you is that they are both very good programs. From what I have experienced, you get quite a bit more .pdf support from TalkingBass. With both programs, you'll probably want to consider getting some free notation software [MuseScore] to supplement the .pdf material provided. The reason being that with the provided .pdf's, you will still have an issue of memorizing (just from sheer redundancy) the provided notation. If I'm not mistaken, Mark from TalkingBass even recommends this early in the course. Phil Mann teaches the sight reading courses at SBL and he is an excellent instructor.
With the TalkingBass courses, you have lifetime access to the course. With an SBL membership, you have access for a year (unless you get a lifetime membership). But once you know it, will you really need lifetime access?
Purchase a course at TalkingBass and you get that course. Purchase an SBL membership you get access to all of the courses available to members. In addition to sight reading, there are also courses on theory, chord tones, harmonic layering, and groove just to name a few.
Mark has a couple of YT videos that are excerpts from his sight reading course. Check those out. And at SBL, you can get a free 14-day trial. If you have not already done so, get the free trial and check out the sight reading courses (and others) for yourself.
The question becomes, "Do I need more .pdf support?" or "Do I want access to more than just sight reading courses?" And only you can answer those questions.
Every now and then, Scott will offer lifetime memberships at a substantial discount. You might start off with an annual and when he offers the discounted lifetime, be ready to pounce!
Both courses will help you become a very proficient reader.
Best of luck!