I shoot on a t4i and currently own just two lenses - the Canon 85 1.8, and the Canon 40 2.8. I generally use the 85 outdoors and the 40 indoors, and that works pretty well for me.
The quality difference between my two lenses is pretty big though. The bokeh on my 85 is awesome, and it's also super sharp. I know this is a little biased because I generally use it in better lighting conditions, but I think it's objectively a much better lens. I'd like to get something just as good for my indoor uses (parties, pictures of my 1 year old, etc).
One option would be the Sigma 35mm 1.4, which is at the very top of the price range I could consider ($900). Coming in a little less would be the Canon 35mm 2.0. Would I notice enough of a quality upgrade going from the 40 to the 35? Anything else I should be considering?
I know it wasn't one of the requested options, but, if you're willing to spend $900 on a new lens just for indoors, get a 6D on sale and keep using the Pancake (I love the Pancake on my 5D2). The 6D will give you 1.3 stops extra low light performance with the same lens, bring new life to every lens you have now and get from then on, and you have all the benefits associated with having a high end camera (mild weather sealing, AF microadjust, custom settings on the mode dial! wireless connectivity, have you seen how long the battery lasts on the 6D? Shall I go on?).