I have both and there is almost no similarity in use. The 75/1.8 is used when I know I'll need a bright mid tele, such as concerts, shows etc. Aslo I also like to shoot primes when I fancy the challenge of a single focal length to help develop my seeing eye (and trust me, it needs help).
The 35-100/2.8 is a walkabout lens for when I expect to need a variety of focal lengths from shot to shot. I went to temple fund-raiser on Friday and took the 7-14/4, 20/1.7 and 35-100. The temple was dull, essentially still in the planning stages and nothing to photograph. So I took a walk around the surrounding bushland and kept the 35-100 on the whole time:
Here's a few images from the walk, not as a recommendation for the lens, just an example of the varying FLs on the walk:
100mm
93mm
68mm
35mm
46mm
100mm
100mm
For the last two, even the 100mm wasn't enough and the images are cropped.
I like having both lenses. If you feel you may eventually want both lenses, my advice would be to go for the 75/1.8 first for the following reasons:
- If you go for the 35-100/2.8 first, it will be a lot harder to persuade yourself that the 75/1.8 is worth the money, just for the extra stop (and a bit).
- The 35-100/2.8 may be a bit overpriced at the moment. Olympus are rumoured to be announcing some high quality zooms soon. This will possibly give you extra choice, and may also see the price of the Panasonic losing some of it's premium.
However, if it is definitely going to be one or the other, I'd say go with the 35-100/2.8; it's very good and very versatile.
Optically, the Olympus has more resolution. In real world shooting, the differences are quite hard to spot, in fact, in real world shooting, very hard. But they are there if you peep and compare very carefully. In fact, I'm pretty confident the 75 is a little held back by the current sensors and will deliver even better results when the next round of sensor improvements are available, whenever that happens.
This is just opinion and guesswork based on what I think I see, nothing scientific. To some degree, all lenses will improve because resolution loss is in part down to the sensor efficiency, and better sensors will improve that. But there comes a point where optically the lens reaches its limits and sensor improvements won't bring any further gains, and I think the 75/1.8 will be one of the last of the current lenses to reach that limit.
-Najinsky