Pretty much all the recent Rebel/60D/7D share the same basic sensor, so RAW performance will mostly be similar. The 70D with its first real new APS-C sensor from Canon in a long time. Unfortunately, from my brief looks at the DPReview comparison shots (select the RAW, and then ISO you want to compare), it seems like it's about the same as the 7D, give or take a bit. Add in even the old 5d2, and you'll see an immediate difference at high ISO.
A year or so ago, I had a 5DII and 7D, and I bought my 5DIII. The quality of the full frame sensors, even in good light, is so much better that I sold my 7D soon after. I shoot wildlife, so giving up a crop sensor was tough, but the files are so much better I haven't regretted it. I use my 1.4x a lot more though
You can probably pick up a used, refurbished (check Canon & Adorama) or even brand new 5DII for the cost of a 70D and if you shoot in low light, you'll be glad you did. Yes, AF isn't great, but it's as good as any Rebel and if you use the central sensor and focus and recompose, you'll be fine.