I have a Maverick and it is a good unit, however, I don't use it any more. If you like buying toys, go ahead and get a Maverick. If you want simple or to save a few bucks, I would do the following:
First, I would test the thermometer. Boiling water is about 212°, take out the thermometer and put it in boiling water (full rolling boil) and see how far off it is. If it is really far off, Weber should replace it.
Second, realize that other things affect the temp reading. My belief is that the Weber thermometer's stem is too short and the reading up that high in the dome is affected by air flow. Try turning the lid so the vent points in another direction (away from the direction of the wind) and see if that changes anything.
Third, you've already realized the easiest, least expensive thing. Just adjust to what you have. If you're reading is always off by about 30° you add 30° to your reading and you're all set. Given that your ribs were done in four hours, I'd say your thermometer is off by about 75°. Generally, at 250°. baby backs would take about six hours (assuming you did not foil).
Anyway, just something to think about. Like many folks, I bought a lot of things out of the gate. Currently, I only use my Thermapen and a cheap thermometer I installed (I have an older unit). I will say out of the things I bought, the Maverick did get a good deal of use. However, after awhile, the WSM is so predictable that I didn't need it. I can easily get my WSM going and once it's stable (about 1 - 2 hours), I can leave it for around 8 hours without worry (i.e. I can sleep on an overnight cook.).