Hi,
I recently shot my 5DII for exposures of over seven minutes at ISO 200, taking landscapes by moonlight along Big Sur, Calif. The temperatures were around 45 degrees F and I mention that because I have read that the ambient temp can be an important consideration (cooler is better). I shot RAW + JPEG and will post a JPEG here later as I am not at my regular computer. Here are my thoughts about the process on the 5DII
QUALITY I thought the image quality results were quite amazing but have not pixel-peeped the image at 300% etc. The camera seems to work very well for such exposures and is head and shoulders above film from what I can see. I would be totally confident based on this to go for exposures up to ten minutes without hesitation. This is not to say the camera can't go much longer, I just have not attempted one hour exposures yet.
BATTERY As for battery life, I shot about ten of these exposures during the evening with the battery showing more than 50% when I was done. I believe I could have gotten a one hour exposure or even much longer without any problem. If you have the grip with two batteries, you probably get two hours or substantially more.
LIVE VIEW Under these admittedly extreme conditions, which it sounds like you will be encountering, the live view was not very helpful in composing the scene as there was just not enough light for it to gain up and show a useful image. I prefer composing with live view but was very glad to have the OVF which was actually useable to show the broad compositional elements of the scene, but not for showing fine details. You will have to fine-tune composition around the edges in post, as even the OVF will hardly show details at such low light levels (maybe also my 50 year old eyes have something to do with that!).
For shooting more normal scenes, live view is a completely decisive advantage and there is no question about it. Other posters have already gone over this.
MIRROR LOCK People, including Michael here, have made much ado about Canon's poor mirror lock but I think they are way off base at least for the 5DII. You push the live view buttion and presto: instant mirror lock! It is a dedicated one-button one-push mirror lock button and end of the problem. I can't comment if Sony's is any easier, but it is hard to imagine anything can be easier than Canon's implementation.
I won't go over the other pros and cons you mentioned in the opening post except to make one comment--you should not, ever again, invest in an expensive camera that does not offer full HD 1080p video. I am a dedicated still photographer for 20+ years but found it a revelation to shoot wildlife video, crashing waves, trees full of moving monarch butterflies, etc. The ability to switch instantly from video to still and back without changing cameras was just amazing. It took my ability to capture the visual beauty of our world to a different level and you are really depriving yourself of some joy if you forego it. The HD video of the 5dII is completely stunning in its quality, and tripod shooters will really get the best results here. I can not only get huge prints but also wonderful multimedia shows to play on my HD television all with a single tool.
Hope this helps!
Guy