I have not used the RXII, we in america are not fortunate enough to. But judging by the number, 20% seems significant at first, but on a second thought it really isn't. If we round up to 25% that is only 1/4 stop. I used my RX and stopped down from 2.8 to 3.0, which is 1/2 stop and noticed a VERY minute change, but it was noticeable. I am not certain that a 1/4 stop difference would be noticeable at all and I very much doubt it would be usefull.
I obtained a used/demo RX for $499 at BH complete with original warrenty, box, accessories and manual. While I know that finding a used or demo RX is mostly luck, and this was the only one listed at BH at the time, if you can find an used RX in 9-or-greater condition, and are willing to buy used, I am not certain that a less than 1/4-stop difference is worth the added $$.
I have read in another post that grey market RXII's in the US sell for $780. "Official" RXII's we can assume would cost some more, but still this is not an expensive camera by any means, but is $280+ worth a less than
1/4 increse in the viewfinder? Maybe for you it is.
If you are planning to buy new, I am not sure what features from the RX did not make it to the RXII. If the DFI is one of them, and this would explain a brighter screen from what I understand, I am not sure that $380 more for a new, old stock RX (based on Calumets price of $999) is worth the DFI, it does come in handy don't get me wrong, but it was a pretty big disapointment because I was expecting it to be more useful than it is. If you plan to shoot with a screen without microprisms or split screen, I think i'd use it all the time, well, when it works which is probobly only 65% of the time.
However, if the DFI is faster and more reliable in more situations on the RXII, and you control DOF religiously, I would suggest take gamble with a 380$ bet and give it a try, there are situations where it is useful. Like when you you are photographing something and don't have enough light for a small aperature but do have enough for a moderate and focussing somewhere in the middle to produce a sharp image. It's a nice secondary reference from DOF preview, which can be kind-of-hard to read in 35mm photography.
Either way, the RX/II seems to be a good alternative to the RTSIII if you don't need/want that much of a camera or cannot afford it. The ST seemed kind of pricey for as few added features over the RX, though, a 1/2-stop faster shutter might come in handy for those wide-open shots on sunny days. How many years have I used cameras without an illuminated counter? LOL, hardly worth teh differential.
The AX I just don't trust, WAY too much mechanics, and the Aria is too light.
But I am NOT a contax expert, even though i played one here, I have done extensive research on the models other than the G- and N-series. I have little first-hand experience and have been shooting contax for a whole, umm, two days now. ::

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Either way, you can bet that this viewfinder won't be as bright as the Nikon FM's was which was by far the brightest I have ever used, obviously I prefer zeiss optics over the bright viewfinder of the FM and the average optics of Nikon.