I had a D700 with battery pack to replace a D3S with a more-versatile setup (I could remove the extra grip). The D3S has better handling - so much better that I loaned my D300 to my sister and bought another new D3S. Your milage may vary.
A friend of mine has a quick evaluation, "Are you looking for a reason to say 'yes' or a reason to say 'no'? It sounds like you are looking for a reason to say 'Yes!' so my advice is buy and try. The resale on a used DLSR is lower but to really give it a try you need to use it for a month or two. The lower resale is still better than the cost of renting for that long. Renting is just teasing yourself and a waste of money if you are truely looking for a reason to say 'yes'.
I loved my D300 but the newer cameras handle a LOT better in darkness. The D3S ISO is awesome. The D4 focuses better in darkness so I'd go directly to the D4 if price isn't an issue.