Depends on what you want from the phone, from what I can see the differences are storage capacity, screen quality and size. Basically from what I can see:
HD7 - Larger screen, double the capacity (16G) with maybe the ability to expand!
Omnia - Better Screen (for Contrast/Colours), smaller dimensions, faster memory (Nand), better design (subjective!)
I have the HD7 and tbh I am finding it just a little to big for me, and while the screen is good I don't think it is anywhere close to the SAmoled. I have an Omnia on it's way as I had to renew my wife's contract and got a brilliant deal. My HD7 is a work phone so I have nothing to lose if I use the Omnia 7.