The first season of Index is a teensy bit formulaic at times. Arcs are split into 4 episodes each (with one or two "extra" episodes in between), consisting of: Touma encounters villain with evil plot, villain throws shit at Touma, Touma closes the gap to punch villain in the face, rinse and repeat. Also Index bites Touma sometimes but very occasionally does something interesting or useful. Not a lot of character or setting development, generally speaking, but enough to get by on. It also manages to avoid being even a pseudo-harem by having a reasonably large cast of relevant male characters like Smoking Fire Mage Guy, Kung Fu Guy, and possibly the greatest character ever, Accelerator. Unfortunately, while there is a reasonable amount of development for side characters, Touma himself doesn't get a whole lot besides the inherent badassery of beating enemies with supernatural powers via punch to the face.
Railgun is a lot better when it comes to developing the setting and characters, but it comes at the cost of having decidedly less in the way of plot, to the point where I'd almost say its more "slice of life" than anything else. Also, for various reasons, the plot can't have the main character actually encounter anyone stronger than her up until it intersects with Index, so most "fights" consist of "Watch Misaka effortlessly crush her opposition!" or occasionally "Watch Kuroko crush her opposition with a small amount of effort!". I think there were maybe two instances through the whole show in which a main character was even theoretically at risk of losing a fight, so it began to suffer from a similar issue to the one that 00 Gundam had. Still a worthwhile show, but don't go into it expecting Index (or vice versa).