I have SLX, XT and Saint. All perform flawlessly, as I’ve come to expect from Shimano at this level. The XT and Saint look more bling, have ceramic sealed jockey wheel bearings rather than the usual bushings on the SLX. The SLX parallelogram is partly steel, whereas the XT and Saint are all alloy, making them slightly lighter. The Saint is stronger.
For my money, the XT and SLX are much of a muchness, just get whatever matches your cranks. The Saint, on the other hand, is a different animal – both in looks and toughness (and price!), and may be the way to go for the style of riding that suits a double & bash up front.