third try to get answer

What do you want to know? Which system has the lenses you want? Which is more important to you, wideangle architecture/landscape, or telephoto sports/wildlife? If you are going to consider the D300s then the 7D deserves a look.

That's far too cryptic and non-specific a question to get any sensible answers to, Jay.

How's this? The D300s is great for stuff it's good at, the 5D Mk II is great for the things it's good at.

Oh - and I'd have a 7D over either of 'em...

Jay: 5DMKII has a different (larger) sensor format (it's the same size as 35mm film). The Nikon has an APS-C format sensor.

I'd use the Canon for low-light weddings and the Nikon for high-speed sports and similar action where a really agile AF system is necessary. But these two cameras require very different choices of lenses. Nobody can help you unless you at least do us all a favor and mention how you think you'll be using the equipment, what sort of budget you have in mind, etc.

Are you trying to choose between these two as used camera purchases, or as new equipment? Both are getting older, now. For many people the D300 series is better replaced by the newer D7000 (again, depending on how you're going to use it). But as mentioned above, if you don't help people to help you, you're just going to get more-or-less random observations (like mine, just now).

Asking for a comparison of these 2 cameras here is like going on a BMW forum and asking for a comparison of a 740i and a Chevy Corvette. Most people who have a lot of experience with one don't with the other although we have all heard of both.
What is most useful when navigating these forums with a question is to narrow the question into bite size chunks: For example: I have these lenses for ____ system and I shoot (weddings, babies, trees, birds, Nascar) and I'm hoping to accomplish ______ with a new camera.
What you have asked is "do you walk to work or carry your lunch"... :)
Really.. what you are asking is 1. Canon vs. Nikon 2. full frame vs crop
Enjoy the hunt.
Richard

As implied by the answers already, sometimes when you don't get an answer, it's because something is wrong with the question.

Take the advice above on how to proceed.

In general, choosing a camera made by one manufacturer over another (especially Canon and Nikon) is a matter of style and feel. Most comparable cameras by either will function essentially the same. When you compare different kinds of cameras by either, then it gets even more complicated.

Rather than asking such a monumentally vague and general question ("what do you think...?"), how about a different approach?

Since you have "done a lot of research," you must have come to some conclusions or developed some understandings about these two cameras. Yet, some things must still be a bit confusing or unclear, even after this research.

To avoid non-answers and to focus the responses, how about asking some specific questions related to your conclusions based on your research or about how and whether what you have learned might make a difference in your choice?

As currently worded, your question is roughly equivalent to: "I'm thinking of going to the store and trying to decide whether to walk or drive. I understand all of the facts about these options. What do you think I should do?"

Dan

I prefer the 5D2 over the D300S. Why? I like the IQ of FF for making large landscape prints. Plus the bigger VF is easier on the eyes and faster to compose with. Many of the Nikon controls and exposure algorithms seem ass-backwards but that's because I've gotten used to the Canon. Not that I have anything against Nikon. I still heart mah FM3A and Nikkors.

Really, you should decide whether you want a DSLR with a full-frame sensor or a crop sensor before you worry about specific makes and models. There are pros and cons either way, and importantly, which way you go will greatly affect your choices in lenses. Threads here on the 'DX vs. FX' question, while focused on one brand (Nikon), largely address this question in a broader sense that applies to Canon as well. You may want to research those threads before you worry about specific makes and models of camera.

If you are still set on just looking at these two cameras, the well-known site dpreview.com has reviews on both. The 300S review has 31 pages and the 5D Mk II review has 41 pages. Both reviews were written by people who spent a great deal of "hands-on" time with the cameras. Each review even has a conclusion where they summarize their findings and weigh the pros and cons of the camera they are reviewing. You will get far more information from these reviews than you will get from any responses here to your vague question.

After you have viewed all 72 pages of the above reviews and considered them, you may wish to come back here to ask focused questions regarding any issues that are still not clear to you. If 72 pages is too much to assimilate, just read the conclusions. However, no one here can make your decision for you and it's not like one camera is all-around superior to the other one in some sort of abstract sense. In certain specific uses or attributes one camera may be superior, but you have provided no information about how you intend to use the camera, what lenses you already have, or what your criteria are, so we have no basis for providing targeted and relevant information to you.