Heya,
If you want more isolation in environment, you want telephoto. Shorter focal length includes more environment, so a 35mm even at F1.4, will still include more background information, and keep it more in context, it's great for telling a story or giving a photo a context. Telephoto focal lengths destroy the background, so you just get isolation, and remove the background and keep it just like a blurry backdrop.
APS-C sensors have you standing further away for the same focal length used for the same frame up field of view as full frame. Because you're further away, you get more depth of field. This means even at F1.4, the depth of field "look" will be a lot more like what F2 looks like. My point is, if you're shooting F2.8, it will look more like what F4 looks like in terms of depth of field on a full frame camera. So if you want to get maximum isolation, get an F1.4 lens.
If you're doing environmental shots, I would not use the 35mm. I shoot with 35mm and 85mm on APS-C often for portrait. The 35mm is great, again, for context shots. But telephoto is where you want to be for the maximum isolation environmental portraits.
You already have a 100mm F2, so you have that.
The Samyang 35mm F1.4 is what I would get, if you want 35mm F1.4 on a budget. I shoot with the 35mm F2 IS and love it. But if you want minimum depth of field, the F1.4 is the way to go. That lens is sharp and it has great bokeh quality. If you want autofocus, you pay for it, and the Sigma 35 ART is the way to go. There's very little reason in my mind to even consider the 35L (the 35 F2 IS optically is about where it is, and better in some areas; and the 35 ART bests both of them).
I like the separation that 85mm F1.4 does.
IMG_3361 by
Mwise1023, on Flickr
IMAGE LINK: https://flic.kr/p/nXMk7w IMG_5370 by
Mwise1023, on Flickr
Personally I would get the Samyang 35mm F1.4 for portrait. I manually focus at F1.4 regardless of having autofocus anyways, so I don't see a reason to pay more than double for autofocus ability. I have my 35 F2 IS for that. But if i wanted maximum isolation at 35mm, that's the one I'd get. Otherwise, I would consider the Samyang 24mm F1.4 on APS-C too as a neat portrait lens with some minor distortion like effects. I would also consider the Canon EF 28mm F1.8 lens for APS-C too. All of course transition to full frame.
Very best,
