I went for the Silvestri, because of its availability and that it's relatively small and light. I rather have two smaller batteries than one big so I can have one at home charging, or have a (warm-kept) backup in the field.
Almost the exact same product can be bought off ebay for like $30 from china, but you never now what type of battery quality you get then, and delivery time can be looong. My experience with such products is that some are good and some are not so good and you never know in advance. As I work in cold temperatures sometimes I need good quality of the battery.
Concerning DC to firewire 800 cable I got the Silvestri too (which looks kind of home-made and unnecessarily thick and inflexible), because of immediate availability, but I will probably change that later to a Leaf DC-FW800 L-plug (if I can get hold of one) sitting in the back and a flexible thin DC-to-DC cable (maybe coiled) for the battery. I don't like the idea of having a thick inflexible excessively long cable hanging around in limb-catching loops :-).