Thank you for your insights into your use of the 6x9 Arca Swiss. Obviously from your point of view the 6x9 works well for your type of working style.
My point in this thread was to solicit objective and varied opinions. I believe that I will not give up 4x5 as I love the shooting style that I have developed with it. Additionally, with the introduction of the new single sheet T-Max 100 Readyloads (my film of choice since Tri-X is not available in Readyloads)I have been getting excellent exposures without light leaks.
So my point was to ascertain if shooting 4x5 vs 6x9 with todays film emulsions and digital workflow will allow comparable results with image enlargements of a reasonable size (perhaps up to 20x30). Your opinions regarding this point are very helpful and appreciated.
To elaboorate; I recently shot 5 rolls of Agfa Scala on my Contax 645. I drum scanned a number of these images and I was quite taken and impressed with the results. Tonal range, detail, sharpness was all excellent. I print to 20x24 on my Epson 7000 and found the difference between 4x5 T-Max and the 645 Scala to be subtle and difficult to detect visually. So I would think that the difference between 4x5 and 6x9 might be evel less so.
Thanks all for the great response and gamut of opinion.