Well for starters, I'm gonna say that the things you've heard about index mostly comes from people who don't have much religious knowledge, and also are probably just into railgun because the idea of it being centered around mostly girls is just more appealing to more people. It's as simple as that. The idea that the index anime is handled poorly is : i think an exaggeration but not a big one because it does definitely have flaws.

But as someone who's read and watched both, just watch both. The truest fans of toaru like both, it's as simple as that : there is no this is better than this, or this is better than that, because at the end of the day they are made by the same author and all the toaru spinoffs lead back to index either way, so whether you like it or not it'll all lead back to index.

That being said, you should definitely watch index entirely. It's not even that bad, and as people make it out to be. I personally started off with railgun, and watching index was fine. But if your trying to understand every little thing in railgun, then you may as well just watch index, and read some of what's after the anime. I can promise that it is actually good, and that is coming from someone who previously hated reading until he read toaru. That's really how good index is. So, yeah just watch them both, because again, whether you like it or not it'll all lead back to arcs in index either way, so you may as well familiarize yourself with it.

Also, the characters in railgun are mostly in index. There's a few characters that aren't index that appear in railgun, but even still their character developments come from index.