[carrrlangas]

I can get ~1 2/3 stops lower ISO compared to the 35/2.4.

1 and a half actually. And then you´ll have to deal with thin DoF. I rather higher ISO noise than out of focus blur.

Here´s my point of view:
Take a look at the EV values of indoor lighting:
Exposure value - Wikipedia, the free encyclopedia

Here´s a crappy copy paste of the table:

Galleries 8–11
Sports events, stage shows, and the like 8–9
Circuses, floodlit 8
Ice shows, floodlit 9
Offices and work areas 7–8
Home interiors 5–7
Christmas tree lights 4–5

So, lets take the worse case assuming we have EV 6 indoors. Now look at the EV values corrsponding to different aperture and shutter speed combinations:
Exposure value - Wikipedia, the free encyclopedia

Although teoretically incorrect, this table corresponds to ISO 100. If you look at EV 6, you´ll find this values:
f/1.4 1/30sec
f/2 1/15sec
f/2.8 1/8sec
f/4 1/4sec

Moreover, at 35mm, f/1.4 gives a DoF of 30cm at 2,5m distance to subject, while at f/2.8 you get 60cm and by f/4, 84cm

If we use ISO 800 (limit for current 16Mpx sensors before noise starts to appear) we get this (adde DoF "column" for a subject at 2 meters):

f/1.4 1/250sec 18cm
f/2 1/120sec 26cm
f/2.8 1/60sec 37cm
f/4 1/30sec 53cm

I find f/2.8 and f/4 values acceptable. And primes that speed will be smaller and lighter. Just make sure the prime has good IQ wide open, like the DA35, F28,

[Docrwm]

Would you say it is worth the extra cost?

They are only available now used, they were discontinued when they started their new three line approach with Art and other lenses. The only ones I know of are refurbished out of Japan at around $450. You can get a 35/2.4 for $199 new and around $150-165 used in great shape.

[Kozlok]

I included the difference between 35 and 30 to allow for slower shutter speeds, which accounts for the extra 1/3

[Drake Avenue]

I don't use my DA 35 since I bought my Sigma 30mm, though the DA is sharper in the corners. I've found that with the portraits I shoot, the corners are not very important.