Railgun will be quite different but probably still good if you like the characters and Index 2 will probably be more of the same so it should all be good as well.
The Railgun spinoff I liked, as for the original index anime I didn't. The index story or it's character's didn't really hit the spot for me, railgun was a comedy though. So the story didn't matter too much, and there was less fanservice too in railgun which helped since I'm not too fond of such content.
I just finished watching the first three episodes of Railgun, and I'm pretty sure I saw more fanservice content in the first two episodes of Railgun then all of Index combined. :/
I have a feeling I'm going to like Index more then Railgun, although Railgun is looking pretty good so far. The action scenes are pretty awesome, but the overall silliness is something I'm not so fond of. Kuroko's character is starting to annoy me already (with her pretty much molesting Mikoto all of the time), but the other characters are alright so far.
_________________ The Local Video Gaming Lunatic
Recent stuff I'm into:
Watching: A Certain Scientific Railgun (ep.14, stalled), Hoshi no Kirby (ep. 71, stalled), Nazo No Kanojo X (ep. 8), Acchi Kocchi (ep. 8)
Reading: nothing