BZ2 actually had the scrap too. I remember there were one or two missions where no biometal pools were available and you had to rely on scrap. In general though, I liked the BZ2 resource system better. The scavengers had an annoying tendency to wander into enemy territory to collect scrap, and due to the first-person view it was much harder to keep track of them than in a typical RTS. (sending an escort unit with them was fairly useless since the BZ1 unit AI stopped defending itself when it was ordered to guard something else)
I thought BZ2 was a substantial improvement in most aspects of the gameplay and interface. They introduced control groups in it (which also allowed you to build a lot more units at once) and the radar station with the traditional RTS view was pretty useful for setting up formations and coordinating multiple attacks at once. The unit AI was somewhat better and your units could fight more effectively on their own, with less of the constant micromanaging that was needed in BZ1. They also fixed some balance issues from BZ1, like some of the more expensive units that were basically useless due to a low ammo capacity that the AI couldn't handle well. There was also a larger selection of weapons and other upgrades available, although most of the weapon sounds were pretty crappy compared to the BZ1 ones.
I did like the BZ1 plot better though. BZ2's story didn't make much sense early on and had a number of apparent plotholes, although it got much better towards the end of the game.
A sequel that expanded on the cold war themes would have been great. Maybe US, USSR, and PRC?
There was in fact a BZ1 expansion pack called Red Odyssey that had a Chinese faction in it. I've never played it though.