Personally, I think Stewart is complete rubbish. It exemplifies the type of "paper mill" textbook publishing which seems to be common these days. It seems he puts out a new edition every few years, and university students across the country are then forced to cough up the $100 for a new copy. The book seems very watered down, and upon reading through it I felt like the target audience was med school prospects, rather than physicists or mathematicians. (Now that's fine, if you're one of those then it's probably perfect!)
That said, it does seem to be the standard text for freshman calculus courses, so clearly somebody likes something about it. Comparatively, Spivak really does not seem like the type of book which should be used lightly, as it assumes a bit more mathematical maturity than most students seeing calculus for the first time have.
A final thought: I don't know what your purposes for learning calculus are, but the two books rather diverge there. Stewart is likely sufficient for a physical sciences undergraduate, and 3/4 physics majors. For those 1/4 of physics majors who are more interested in the mathematical theory, Spivak is a good bridge into upper division mathematics (real analysis, chiefly. But more than just that it serves as an introduction to rigor). For mathematics majors, Stewart is fairly useless since the majority of Stewart is examples in how to compute derivatives or integrals, and mathematicians do not really concern themselves with these trivialities.