I've had both and I preferred the 30. I recall that it had video toggle for rec709 and sLog monitoring switch right on the EVF itself rather than having to assign it to a button on the body or fish through menus.
I believe the C30W came out first. It was the first HD Sony colour finder. There were however many issues, the main one being it got zapped by the Sun very easily.
Sony discontinued it and totally redesigned it - coming out with the C35W. In Sony's eyes this was a much better unit. It has a 3.5" flip out screen that could be orientated most ways you would need it and worked well. However, the unit itself is rather large and there were many issues with clearance on larger lenses and also clearance of matte boxes when using with shorter lenses (eg. Zeiss Standard Speeds).
So, Sony brought out the C30WR - it was the same housing and basic design as the C30W, but a totally new panel which was a bit more resistant to Solar damage, a new image processor, extra buttons relating to LUTs and a few other things.
In practise the C30W and WR provide you with basically the same image, but the WR commands a higher price, rightfully so.
I have had all three at one point in time. None are spectacular.
They all share the same standard sony HD viewfinder connection, as used on the earlier B&W HDVF-20A. All of these finders are fed an ANALOGUE signal, which keeps the delay minimal.
So what are you guys doing, saving up for a zacuto gratical or using one these massive things? I personally hate adding something that needs more batteries. From the specs I assume these look about like an Alphatron? If so, not ideal but it could be worse.
Do they have zebras? How are you judging exposure with them?
I have the OLED finder for the F65, but the C30WR is the best all round finder available for the F35.
I spoke to a guy recently who bought a Zacuto Gratical and had to return it - he had a lot of problems with the image being very inaccurate. Not sure if it was down to a dodgy batch but it went back to Zacuto.
The C30WR is great for judging exposure. You can do it just by eye if shooting in a 709/LUT mode, or with customisable zebras (two available).
The image quality on the 35 is inferior to the c30wr. Milky blacks.
Carrying an extra battery and arm when you can plug an original EVF is priceless. And not mentioning the lag on external EVFs.
The only complain with the C30WR is that the magnifying glass is too small, therefore the image is a bit small. I tried adapting a Zacuto EVF loupe in the sony EVF and it was FANTASTIC. Unfortunately no way to make it work since the Sony EVF needs a 45º angle mirror
I bought a second F35 JUST for the C30WR (financially it was totally worth it, plus I have two F35s now ) I have to say it's easily the best viewfinder situation I can imagine on the F35. I use it or the O7Q religiously depending on if I need a VF or a Montitor.
As James said - you can trust it's rendition 100% and it's great for judging exposure, and it's exposure tools built in suit the F35 VERY well.
I would go for the C30W unless you had very good reasons for wanting the 35W.
The 35 gets in the way of a lot of lenses, both large and small (large ones it hits the lens, small lenses it will hit the matte box).
The only real advantage is the 35 can be flipped out and you can look at it like a regular LCD, where the LCD panel on the 30W is fixed but still versatile because you can look at it without the loupe on, and without the mirror on if next to the camera (great for tracking).