7d+17-55 and 5d+70-200: does-it make any sense ?

[unkbob]

You say you have "several lenses" for your 7D. Are they all crop-only or will they work on the 5D3?

I have the 100L macro and the 70-200 4L IS (I know it is redundant with the 70-200 2.8L IS but I could not convince myself to sell it when I upgraded to the 70-200 2.8L: it is razor sharp and its light weight makes it a really nice lens for sunny day hikes)

I do really need a wider lens in my kit : maybe the 30 1.4 sigma + 50 1.4 : not too much money to put on the table once my 17-55 is sold.

Just bear in mind the Sigma 50 1.4 is FF, the Sigma 30 1.4 is crop only. Both great lenses, but I don't see the point of the 30 1.4 on your 7D when 50 1.4 will give you similar FOV and much better high ISO performance and DOF control on the 5D3.

I think you're bonkers to keep the 70-200 F4 IS at the expense of wider lenses, given your situation. If you want something long and light, you have the 100 L (at either 100 on FF or 160 equivalent on the 7d), and the 70-200 2.8 is there when you need it. I am biased as I love wide lenses, but something like the 35L on your 5D3 will blow your mind. Even the 24-70 is amazing on FF - you get similar shallow DOF with a 2.8 zoom lens on a FF as a 1.4 prime on a crop. And when you get up close to a scene, you have more control over composition and the pictures are more exciting, as a general rule. It depends on what you're shooting of course, and YMMV.

Just my 2c