That's a very good question. In truth I don't need a DSLR at all.
The camers has long surpassed my ability to shoot, and even further surpassed my abilities with Photoshop.
But photography is one of my favorite hobbies, and I enjoy experiencing each improvement and incremental feature that comes out..
It seems like having an 800 and a 700 would automatically be better than just 2 700s for a guy who feels a need for 2 dslrs.
I would understand the question better if it were about moving from D700 toD800.
D800 seems way more flexible in resolution, delivering more dx resolution that the d300 has at all. Allowing continued smaller file sizes as an option. The 2card drives, and movie mode are considerable feature enhancements too.
OP .... Why are you unsure of this decision? Don't mean to ask as a challenge, rather to understand better.