I think this answer is a bit more complex than simply answering "body vs lens".
If you are talking about a Canon 70D with a 50mm f/1.8 vs. a Canon T3i with a 50mm f/1.2L , then the answer will be LENS.
But, if you are talking about a Canon T3i with a 50mm f/1.2L vs a Canon 6D with a 85mm f/1.8, then the answer will be BODY.
While the LENS has a huge impact, so does the BODY. But, the biggest impact the body has is FULL FRAME vs CROP. So, you are probably better upgrading the LENS instead of upgrading to another crop body. But if you are moving to FULL FRAME, you may get superior image quality by upgrading the BODY instead of the lens.
Been there, done that. No more crops for me. Basically speaking, FF+zoom ~ APSC+prime, but FF+prime level is unreachable for crops. Maybe the new Sigma 18-35/1.8 on crop is close to f/2.8 zoom on FF (still not as good), but it's size (as big and heavy as 24-70LII) and price ($900) puts things back in their places. No free candy there.
On the other hand, any decent modern lens can produce decent quality images. The new EF-S 55-250/4-5.6 IS STM is considerably better and cheaper than the older EF 70-300/4-5.6 IS USM and, in many cases, it's not bad enough
for upgrading to 70-300L.
IMHO, upgrading is not about mixing priorities, "FF vs better lenses" is like "bees vs honey", they should go together. Every L lens deserves a FF body
. Size matters, specially in physics. Bigger sensor will always give you more ... , due to either bigger pixel size or their quantity.